diff --git a/src/string.c b/src/string.c
index f5b62ab..22bf91f 100644
--- a/src/string.c
+++ b/src/string.c
@@ -12,6 +12,9 @@ int find_substrings(const char* haystack, const char* needle, size_t *num_substr
 
     size_t sz_h = strlen(haystack);
     size_t sz_n = strlen(needle);
+    if ((int)sz_h - (int)sz_n < 0) {
+        return 0;
+    }
 
     *num_substrings = 0;
     for (size_t i = 0; i <= sz_h - sz_n; ++i) {
diff --git a/tests/tests.c b/tests/tests.c
index 5424fba..eed95c1 100644
--- a/tests/tests.c
+++ b/tests/tests.c
@@ -285,6 +285,14 @@ void test_string() {
     subs = NULL;
 
     find_substrings("test one two", "nope", &sub_sz, &subs);
+    assert(sub_sz == 0);
+    assert(subs == NULL);
+    free(subs);
+    subs = NULL;
+
+    find_substrings("123", "nopes", &sub_sz, &subs);
+    assert(sub_sz == 0);
+    assert(subs == NULL);
     free(subs);
     subs = NULL;