diff --git a/vlib/toml/tests/burntsushi_toml_test.v b/vlib/toml/tests/burntsushi_toml_test.v
index 50c1fa07f0..2ba69767d7 100644
--- a/vlib/toml/tests/burntsushi_toml_test.v
+++ b/vlib/toml/tests/burntsushi_toml_test.v
@@ -44,7 +44,7 @@ def normalize: sorted_walk(if type == "array" then sort else . end);
 normalize'
 )
 
-fn run(args []string) ?string {
+fn run(args []string) !string {
 	res := os.execute(args.join(' '))
 	if res.exit_code != 0 {
 		return error('${args[0]} failed with return code ${res.exit_code}.\n${res.output}')
diff --git a/vlib/toml/tests/iarna_toml_spec_test.v b/vlib/toml/tests/iarna_toml_spec_test.v
index 722f21e780..40d21878ad 100644
--- a/vlib/toml/tests/iarna_toml_spec_test.v
+++ b/vlib/toml/tests/iarna_toml_spec_test.v
@@ -60,7 +60,7 @@ def normalize: sorted_walk(if type == "array" then sort else . end);
 normalize'
 )
 
-fn run(args []string) ?string {
+fn run(args []string) !string {
 	res := os.execute(args.join(' '))
 	if res.exit_code != 0 {
 		return error('${args[0]} failed with return code ${res.exit_code}.\n${res.output}')