diff --git a/src/mergesort.c b/src/mergesort.c
index 02094b486aeb..4d5bdbd414db 100644
--- a/src/mergesort.c
+++ b/src/mergesort.c
@@ -44,7 +44,15 @@ static int expr_msort_cmp(const struct expr *e1, const struct expr *e2)
 	case EXPR_MAPPING:
 		return expr_msort_cmp(e1->left, e2->left);
 	case EXPR_BINOP:
-		return expr_msort_cmp(e1->left, e2->left);
+		switch (e2->etype) {
+		case EXPR_VALUE:
+			return expr_msort_cmp(e1->left, e2);
+		case EXPR_BINOP:
+			return expr_msort_cmp(e1->left, e2->left);
+		default:
+			break;
+		}
+		/* fall through */
 	default:
 		BUG("Unknown expression %s\n", expr_name(e1));
 	}