diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index 1b96cce..65b6850 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -102,7 +102,7 @@ static int parse_ull_arg(const char* val, unsigned long long* out, const char* o
 }
 
 static int parse_size_arg(const char* value, unsigned long long* out) {
-  if (!value || *value == '\0' || *value == '-')
+  if (!value || *value < '0' || *value > '9')
     return -1;
   char* end;
   errno = 0;
diff --git a/tests/test_client_cli.c b/tests/test_client_cli.c
index 69b862c..d4c314e 100644
--- a/tests/test_client_cli.c
+++ b/tests/test_client_cli.c
@@ -277,7 +277,8 @@ static void test_parse_args_max_alloc_sizes() {
 }
 
 static void test_parse_args_rejects_invalid_max_alloc() {
-  const char* values[] = {"0", "-1", "1Z", "1K2", "18446744073709551615K"};
+  const char* values[] = {"0",  "-1",  "+1",  " 1",   "1 ",
+                          "1Z", "1K2", "1 K", "1\tK", "18446744073709551615K"};
   for (size_t i = 0; i < sizeof(values) / sizeof(values[0]); i++) {
     Config* cfg = config_create();
     char* argv[] = {"fastsync", "--max-alloc", (char*)values[i], "/src", "/dst"};