diff --git a/_docs/mod/core.md b/_docs/mod/core.md
index c705504..60e9020 100644
--- a/_docs/mod/core.md
+++ b/_docs/mod/core.md
@@ -20,6 +20,15 @@ echo [ARGS...]
 
 Displays the string representation of ARGS to stdout followed by a new line.
 
+### error
+
+```
+error MSG
+```
+
+Raises an error with MSG as the given value. This will start unrolling the stack until a `try` block is
+encountered, or until it reaches the top level stack, at which it will be displayed as a "runtime error".
+
 ### foreach
 
 ```
diff --git a/ucl/builtins.go b/ucl/builtins.go
index 4e3b9ad..3587715 100644
--- a/ucl/builtins.go
+++ b/ucl/builtins.go
@@ -297,7 +297,12 @@ func notBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 		return nil, err
 	}
 
-	return boolObject(!args.args[0].Truthy()), nil
+	v := args.args[0]
+	if v == nil {
+		return boolObject(true), nil
+	}
+
+	return boolObject(!v.Truthy()), nil
 }
 
 var errObjectsNotEqual = errors.New("objects not equal")
@@ -1021,6 +1026,13 @@ func foreachBuiltin(ctx context.Context, args macroArgs) (Object, error) {
 	return last, nil
 }
 
+func errorBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
+	if len(args.args) < 1 {
+		return nil, errors.New("need at least one arguments")
+	}
+	return nil, errRuntime{val: args.args[0]}
+}
+
 func breakBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) < 1 {
 		return nil, errBreak{}
diff --git a/ucl/eval.go b/ucl/eval.go
index 237864b..98e352e 100644
--- a/ucl/eval.go
+++ b/ucl/eval.go
@@ -186,7 +186,16 @@ func (e evaluator) evalArg(ctx context.Context, ec *evalCtx, n astCmdArg) (Objec
 		if v, ok := ec.getVar(*n.Var); ok {
 			return v, nil
 		}
-		return nil, nil
+
+		if e.inst.missingVarHandler != nil {
+			dv, err := e.inst.missingVarHandler(ctx, *n.Var)
+			if err != nil {
+				return nil, err
+			}
+			return fromGoValue(dv)
+		}
+
+		return nil, errors.New("undefined variable: " + *n.Var)
 	case n.MaybeSub != nil:
 		sub := n.MaybeSub.Sub
 		if sub == nil {
diff --git a/ucl/inst.go b/ucl/inst.go
index 6332256..5414753 100644
--- a/ucl/inst.go
+++ b/ucl/inst.go
@@ -11,6 +11,7 @@ import (
 type Inst struct {
 	out                   io.Writer
 	missingBuiltinHandler MissingBuiltinHandler
+	missingVarHandler     MissingVarHandler
 	echoPrinter           EchoPrinter
 
 	rootEC *evalCtx
@@ -30,6 +31,12 @@ func WithMissingBuiltinHandler(handler MissingBuiltinHandler) InstOption {
 	}
 }
 
+func WithMissingVarHandler(handler MissingVarHandler) InstOption {
+	return func(i *Inst) {
+		i.missingVarHandler = handler
+	}
+}
+
 func WithModule(module Module) InstOption {
 	return func(i *Inst) {
 		for name, builtin := range module.Builtins {
@@ -92,6 +99,7 @@ func New(opts ...InstOption) *Inst {
 	rootEC.addCmd("break", invokableFunc(breakBuiltin))
 	rootEC.addCmd("continue", invokableFunc(continueBuiltin))
 	rootEC.addCmd("return", invokableFunc(returnBuiltin))
+	rootEC.addCmd("error", invokableFunc(errorBuiltin))
 
 	rootEC.addMacro("if", macroFunc(ifBuiltin))
 	rootEC.addMacro("foreach", macroFunc(foreachBuiltin))
diff --git a/ucl/inst_test.go b/ucl/inst_test.go
index 1da27e5..1faa97c 100644
--- a/ucl/inst_test.go
+++ b/ucl/inst_test.go
@@ -119,6 +119,64 @@ func TestInst_Eval(t *testing.T) {
 	}
 }
 
+func TestInst_MissingVarHandler(t *testing.T) {
+	tests := []struct {
+		desc    string
+		expr    string
+		handler ucl.MissingVarHandler
+		want    any
+		wantErr string
+	}{
+		{
+			desc:    "default - error",
+			expr:    `$bla`,
+			wantErr: "undefined variable: bla",
+		},
+		{
+			desc: "handler - set to nil",
+			handler: func(ctx context.Context, name string) (any, error) {
+				return nil, nil
+			},
+			expr: `$bla`,
+			want: nil,
+		},
+		{
+			desc: "handler - set to a string 1",
+			handler: func(ctx context.Context, name string) (any, error) {
+				return "I am " + name, nil
+			},
+			expr: `$bla`,
+			want: "I am bla",
+		},
+		{
+			desc: "handler - set to a string 2",
+			handler: func(ctx context.Context, name string) (any, error) {
+				return "I am " + name, nil
+			},
+			expr: `$something`,
+			want: "I am something",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			ctx := context.Background()
+			outW := bytes.NewBuffer(nil)
+
+			inst := ucl.New(ucl.WithOut(outW), ucl.WithTestBuiltin(), ucl.WithMissingVarHandler(tt.handler))
+			res, err := inst.Eval(ctx, tt.expr)
+
+			if tt.wantErr != "" {
+				assert.Error(t, err)
+				assert.Equal(t, err.Error(), tt.wantErr)
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, res)
+			}
+		})
+	}
+}
+
 var parseComments1 = `
 	proc lookup { |file|
 		foreach { |toks|
diff --git a/ucl/objs.go b/ucl/objs.go
index e6c0fc0..a167dc0 100644
--- a/ucl/objs.go
+++ b/ucl/objs.go
@@ -648,3 +648,14 @@ func (e errReturn) Error() string {
 }
 
 var ErrHalt = errors.New("halt")
+
+type errRuntime struct {
+	val Object
+}
+
+func (e errRuntime) Error() string {
+	if e.val == nil {
+		return "runtime error: (nil)"
+	}
+	return "runtime error: " + e.val.String()
+}
diff --git a/ucl/testbuiltins_test.go b/ucl/testbuiltins_test.go
index 31eb775..359b7c9 100644
--- a/ucl/testbuiltins_test.go
+++ b/ucl/testbuiltins_test.go
@@ -3,7 +3,6 @@ package ucl
 import (
 	"bytes"
 	"context"
-	"errors"
 	"fmt"
 	"strings"
 	"testing"
@@ -58,13 +57,6 @@ func WithTestBuiltin() InstOption {
 			return &a, nil
 		}))
 
-		i.rootEC.addCmd("error", invokableFunc(func(ctx context.Context, args invocationArgs) (Object, error) {
-			if len(args.args) == 0 {
-				return nil, errors.New("an error occurred")
-			}
-			return nil, errors.New(args.args[0].String())
-		}))
-
 		i.rootEC.addCmd("joinpipe", invokableFunc(func(ctx context.Context, args invocationArgs) (Object, error) {
 			sb := strings.Builder{}
 
@@ -230,7 +222,9 @@ func TestBuiltins_If(t *testing.T) {
 			ctx := context.Background()
 			outW := bytes.NewBuffer(nil)
 
-			inst := New(WithOut(outW), WithTestBuiltin())
+			inst := New(WithOut(outW), WithTestBuiltin(), WithMissingVarHandler(func(ctx context.Context, name string) (any, error) {
+				return nil, nil
+			}))
 			err := evalAndDisplay(ctx, inst, tt.expr)
 
 			assert.NoError(t, err)
@@ -255,7 +249,7 @@ func TestBuiltins_Try(t *testing.T) {
 			try {
 				error "bang"
 			}
-			echo "after"`, wantErr: "bang"},
+			echo "after"`, wantErr: "runtime error: bang"},
 		{desc: "try with catch - successful", expr: `
 			try {
 				echo "good"
@@ -276,7 +270,7 @@ func TestBuiltins_Try(t *testing.T) {
 			} catch { |err|
 				echo (cat "the error was = " $err)
 			}
-			echo "after"`, want: "the error was = error:bang\nafter\n(nil)\n"},
+			echo "after"`, want: "the error was = error:runtime error: bang\nafter\n(nil)\n"},
 		{desc: "try with two catch - successful", expr: `
 			try {
 				echo "i'm good"
@@ -304,18 +298,18 @@ func TestBuiltins_Try(t *testing.T) {
 				echo "wow, we made it here"
 			}
 			echo "after"`, want: "wow, we made it here\nafter\n(nil)\n"},
-		{desc: "return value - single try", expr: `
+		{desc: "return value - single try 1", expr: `
 			set x (try { error "bang" })
-			$x`, wantErr: "bang"},
-		{desc: "return value - single try", expr: `
+			$x`, wantErr: "runtime error: bang"},
+		{desc: "return value - single try 2", expr: `
 			set x (try { error "bang" } catch { |err| $err })
-			$x`, want: "error:bang\n"},
+			$x`, want: "error:runtime error: bang\n"},
 		{desc: "return value - try and catch - successful", expr: `
 			set x (try { error "bang" } catch { "hello" })
 			$x`, want: "hello\n"},
 		{desc: "return value - try and catch - unsuccessful", expr: `
 			set x (try { error "bang" } catch { error "boom" })
-			$x`, wantErr: "boom"},
+			$x`, wantErr: "runtime error: boom"},
 		{desc: "try with finally - successful", expr: `
 			try {
 				echo "all good"
@@ -329,7 +323,7 @@ func TestBuiltins_Try(t *testing.T) {
 			} finally {
 				echo "always at end"
 			}
-			echo "after"`, want: "always at end\n", wantErr: "bang"},
+			echo "after"`, want: "always at end\n", wantErr: "runtime error: bang"},
 		{desc: "try with catch and finally - successful", expr: `
 			try {
 				echo "all good"
@@ -356,7 +350,7 @@ func TestBuiltins_Try(t *testing.T) {
 			} finally {
 				echo "always at end"
 			}
-			echo "after"`, want: "always at end\n", wantErr: "boom"},
+			echo "after"`, want: "always at end\n", wantErr: "runtime error: boom"},
 		{desc: "try with finally - finally result discarded", expr: `
 			set a (try {
 				"return me"
@@ -369,13 +363,13 @@ func TestBuiltins_Try(t *testing.T) {
 				error "bang"
 			} finally {
 				error "kaboom"
-			}`, wantErr: "bang"},
+			}`, wantErr: "runtime error: bang"},
 		{desc: "try with finally - error not discarded if try succeeds", expr: `
 			try {
 				echo "all good"
 			} finally {
 				error "kaboom"
-			}`, want: "all good\n", wantErr: "kaboom"},
+			}`, want: "all good\n", wantErr: "runtime error: kaboom"},
 		{desc: "try with finally with error - successful", expr: `
 			try {
 				echo "all good"
@@ -384,13 +378,13 @@ func TestBuiltins_Try(t *testing.T) {
 				if (eq $err ()) { echo "that's nil" }
 			}
 			echo "after"`, want: "all good\nthe error was \nthat's nil\nafter\n(nil)\n"},
-		{desc: "try with finally - unsuccessful", expr: `
+		{desc: "try with finally - unsuccessful 2", expr: `
 			try {
 				error "bang"
 			} finally { |err|
 				echo (cat "the error was " $err)
 			}
-			echo "after"`, want: "the error was error:bang\n", wantErr: "bang"},
+			echo "after"`, want: "the error was error:runtime error: bang\n", wantErr: "runtime error: bang"},
 		{desc: "try with too many finallies - unsuccessful", expr: `
 			try {
 				error "bang"
@@ -412,7 +406,7 @@ func TestBuiltins_Try(t *testing.T) {
 			} finally {
 				echo "outer"
 			}
-			echo "after"`, want: "the error was error:bang\nouter caught\nouter\nafter\n(nil)\n"},
+			echo "after"`, want: "the error was error:runtime error: bang\nouter caught\nouter\nafter\n(nil)\n"},
 	}
 
 	for _, tt := range tests {
@@ -420,7 +414,9 @@ func TestBuiltins_Try(t *testing.T) {
 			ctx := context.Background()
 			outW := bytes.NewBuffer(nil)
 
-			inst := New(WithOut(outW), WithTestBuiltin())
+			inst := New(WithOut(outW), WithTestBuiltin(), WithMissingVarHandler(func(ctx context.Context, name string) (any, error) {
+				return nil, nil
+			}))
 			err := evalAndDisplay(ctx, inst, tt.expr)
 
 			if tt.wantErr != "" {
@@ -1568,6 +1564,7 @@ func TestBuiltins_AndOrNot(t *testing.T) {
 		{desc: "not 1", expr: `not $true`, want: false},
 		{desc: "not 2", expr: `not $false`, want: true},
 		{desc: "not 3", expr: `not $false $true`, want: true},
+		{desc: "not 4", expr: `not ()`, want: true},
 
 		{desc: "short circuit and 1", expr: `and "hello" "world"`, want: "world"},
 		{desc: "short circuit and 2", expr: `and () "world"`, want: nil},
@@ -1636,6 +1633,31 @@ func TestBuiltins_Cat(t *testing.T) {
 	}
 }
 
+func TestBuiltins_Error(t *testing.T) {
+	tests := []struct {
+		desc    string
+		expr    string
+		wantErr string
+	}{
+		{desc: "error 1", expr: `error "bang"`, wantErr: "runtime error: bang"},
+		{desc: "error 2", expr: `error 123`, wantErr: "runtime error: 123"},
+		{desc: "error 3", expr: `error ()`, wantErr: "runtime error: (nil)"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			ctx := context.Background()
+			outW := bytes.NewBuffer(nil)
+
+			inst := New(WithOut(outW), WithTestBuiltin())
+
+			_, err := inst.Eval(ctx, tt.expr)
+			assert.Error(t, err)
+			assert.Equal(t, tt.wantErr, err.Error())
+		})
+	}
+}
+
 func evalAndDisplay(ctx context.Context, inst *Inst, expr string) error {
 	res, err := inst.eval(ctx, expr)
 	if err != nil {
diff --git a/ucl/userbuiltin.go b/ucl/userbuiltin.go
index a576b45..62ff7c4 100644
--- a/ucl/userbuiltin.go
+++ b/ucl/userbuiltin.go
@@ -11,6 +11,7 @@ import (
 type BuiltinHandler func(ctx context.Context, args CallArgs) (any, error)
 
 type MissingBuiltinHandler func(ctx context.Context, name string, args CallArgs) (any, error)
+type MissingVarHandler func(ctx context.Context, name string) (any, error)
 
 type CallArgs struct {
 	args invocationArgs