diff --git a/common/complete.c b/common/complete.c
index 586a7eb..f9c05b4 100644
--- a/common/complete.c
+++ b/common/complete.c
@@ -88,6 +88,9 @@ int device_complete(struct string_list *sl, char *instr)
 	char cmd[128];
 	int len, len2;
 
+	if (!instr)
+		instr = "";
+
 	len = strlen(instr);
 
 	for_each_device(dev) {
@@ -112,6 +115,9 @@ static int device_param_complete(char begin, struct device_d *dev,
 	char *tmp = cmd;
 	int len, len2;
 
+	if (!instr)
+		instr = "";
+
 	len = strlen(instr);
 	if (begin) {
 		tmp[0] = begin;
@@ -156,6 +162,9 @@ static int env_param_complete(struct string_list *sl, char *instr, int eval)
 	int len, len2;
 	char end = '=';
 
+	if (!instr)
+		instr = "";
+
 	if (eval) {
 		cmd[0] = '$';
 		tmp++;