diff --git a/core/source-files/source-files-tests.factor b/core/source-files/source-files-tests.factor
new file mode 100644
index 0000000000..e5e04c777f
--- /dev/null
+++ b/core/source-files/source-files-tests.factor
@@ -0,0 +1,5 @@
+IN: source-files.tests
+USING: source-files tools.test assocs sequences strings
+namespaces kernel ;
+
+[ { } ] [ source-files get keys [ string? not ] filter ] unit-test
diff --git a/core/source-files/source-files.factor b/core/source-files/source-files.factor
index 36a1806e12..454f148974 100755
--- a/core/source-files/source-files.factor
+++ b/core/source-files/source-files.factor
@@ -44,6 +44,7 @@ uses definitions ;
     \ source-file construct ;
 
 : source-file ( path -- source-file )
+    dup string? [ "Invalid source file path" throw ] unless
     source-files get [ <source-file> ] cache ;
 
 : reset-checksums ( -- )