diff --git a/common/stringop.c b/common/stringop.c
index 8af0d60fa..709be6842 100644
--- a/common/stringop.c
+++ b/common/stringop.c
@@ -258,6 +258,9 @@ int unescape_string(char *string) {
 }
 
 char *join_args(char **argv, int argc) {
+	if (!sway_assert(argc > 0, "argc should be positive")) {
+		return NULL;
+	}
 	int len = 0, i;
 	for (i = 0; i < argc; ++i) {
 		len += strlen(argv[i]) + 1;
diff --git a/sway/commands/output/background.c b/sway/commands/output/background.c
index f65904bb8..5a15ed0fc 100644
--- a/sway/commands/output/background.c
+++ b/sway/commands/output/background.c
@@ -61,6 +61,9 @@ struct cmd_results *output_cmd_background(int argc, char **argv) {
 			return cmd_results_new(CMD_INVALID,
 				"Missing background scaling mode.");
 		}
+		if (j == 0) {
+			return cmd_results_new(CMD_INVALID, "Missing background file");
+		}
 
 		wordexp_t p = {0};
 		char *src = join_args(argv, j);