diff --git a/.gitignore b/.gitignore
index 6793e76..757f032 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,6 +7,11 @@
 *.test
 *.out
 
+# go test sometimes writes a go.sum into fixture directories that have a
+# go.mod even though the test only parses the module file. The fixtures
+# don't need a go.sum.
+**/testdata/**/go.sum
+
 # Editor
 .idea/
 .vscode/