diff --git a/core/action.go b/core/action.go
index 64728a2..7582f54 100644
--- a/core/action.go
+++ b/core/action.go
@@ -134,6 +134,9 @@ func Execute(
 	if len(argv) == 0 {
 		return nil, errors.New("empty argv")
 	}
+	if source == "" {
+		return nil, errors.New("empty source")
+	}
 
 	env = append(env, "STATE_PATH=")
 
diff --git a/core/action_test.go b/core/action_test.go
index 803a3c3..8f001e1 100644
--- a/core/action_test.go
+++ b/core/action_test.go
@@ -72,7 +72,7 @@ func TestExecute(t *testing.T) {
 		}
 	}
 	execute := func(argv []string) ([]Item, error) {
-		return Execute("", argv, nil, "", time.Minute)
+		return Execute("_", argv, nil, "", time.Minute)
 	}
 
 	res, err := execute([]string{"true"})
@@ -89,7 +89,7 @@ func TestExecute(t *testing.T) {
 	assertLen(res, 0)
 
 	// Timeout
-	res, err = Execute("", []string{"sleep", "10"}, nil, "", time.Millisecond)
+	res, err = Execute("_", []string{"sleep", "10"}, nil, "", time.Millisecond)
 	assertNotNil(err)
 	assertLen(res, 0)
 
@@ -102,7 +102,7 @@ func TestExecute(t *testing.T) {
 	}
 
 	// Read from stdin
-	res, err = Execute("", []string{"jq", "-cR", `{id: .}`}, nil, "bar", time.Minute)
+	res, err = Execute("_", []string{"jq", "-cR", `{id: .}`}, nil, "bar", time.Minute)
 	assertNil(err)
 	assertLen(res, 1)
 	if res[0].Id != "bar" {
@@ -110,7 +110,7 @@ func TestExecute(t *testing.T) {
 	}
 
 	// Set env
-	res, err = Execute("", []string{"jq", "-cn", `{id: env.HELLO}`}, []string{"HELLO=baz"}, "", time.Minute)
+	res, err = Execute("_", []string{"jq", "-cn", `{id: env.HELLO}`}, []string{"HELLO=baz"}, "", time.Minute)
 	assertNil(err)
 	assertLen(res, 1)
 	if res[0].Id != "baz" {