diff --git a/qemu-img.c b/qemu-img.c
index 86127f0b1..8bdae6649 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -332,8 +332,9 @@ static int img_create(int argc, char **argv)
     /* Get image size, if specified */
     if (optind < argc) {
         int64_t sval;
-        sval = strtosz_suffix(argv[optind++], NULL, STRTOSZ_DEFSUFFIX_B);
-        if (sval < 0) {
+        char *end;
+        sval = strtosz_suffix(argv[optind++], &end, STRTOSZ_DEFSUFFIX_B);
+        if (sval < 0 || *end) {
             error_report("Invalid image size specified! You may use k, M, G or "
                   "T suffixes for ");
             error_report("kilobytes, megabytes, gigabytes and terabytes.");
@@ -710,8 +711,9 @@ static int img_convert(int argc, char **argv)
         case 'S':
         {
             int64_t sval;
-            sval = strtosz_suffix(optarg, NULL, STRTOSZ_DEFSUFFIX_B);
-            if (sval < 0) {
+            char *end;
+            sval = strtosz_suffix(optarg, &end, STRTOSZ_DEFSUFFIX_B);
+            if (sval < 0 || *end) {
                 error_report("Invalid minimum zero buffer size for sparse output specified");
                 return 1;
             }