diff --git a/src/core/wee-string.c b/src/core/wee-string.c
index 70f595553..2593ed279 100644
--- a/src/core/wee-string.c
+++ b/src/core/wee-string.c
@@ -76,7 +76,7 @@ string_strndup (const char *string, int length)
 {
     char *result;
 
-    if (!string)
+    if (!string || (length < 0))
         return NULL;
 
     if ((int)strlen (string) < length)
diff --git a/tests/unit/core/test-string.cpp b/tests/unit/core/test-string.cpp
index 19e7c89c4..a7bc807de 100644
--- a/tests/unit/core/test-string.cpp
+++ b/tests/unit/core/test-string.cpp
@@ -123,6 +123,8 @@ TEST(String, Duplicate)
 
     POINTERS_EQUAL(NULL, string_strndup (NULL, 0));
 
+    POINTERS_EQUAL(NULL, string_strndup (str_test, -1));
+
     str = string_strndup (str_test, 0);
     CHECK(str);
     CHECK(str != str_test);