diff --git a/tests/suites/test_suite_psa_crypto.function b/tests/suites/test_suite_psa_crypto.function
index ac6746d..773163b 100644
--- a/tests/suites/test_suite_psa_crypto.function
+++ b/tests/suites/test_suite_psa_crypto.function
@@ -13,18 +13,18 @@
  * \param buffer    Pointer to the beginning of the buffer.
  * \param size      Size of the buffer in bytes.
  *
- * \return          0 if the buffer is all-bits-zero.
- * \return          A nonzero value otherwise.
+ * \return          1 if the buffer is all-bits-zero.
+ * \return          0 if there is at least one nonzero byte.
  */
-int mem_is_nonzero( void *buffer, size_t size )
+static int mem_is_zero( void *buffer, size_t size )
 {
     size_t i;
     for( i = 0; i < size; i++ )
     {
         if( ( (unsigned char *) buffer )[i] != 0 )
-            return( i + 1 );
+            return( 0 );
     }
-    return( 0 );
+    return( 1 );
 }
 
 static int exercise_mac_key( psa_key_slot_t key,
@@ -349,8 +349,8 @@
                              exported, export_size,
                              &exported_length );
     TEST_ASSERT( status == (psa_status_t) expected_export_status );
-    TEST_ASSERT( ! mem_is_nonzero( exported + exported_length,
-                                   export_size - exported_length ) );
+    TEST_ASSERT( mem_is_zero( exported + exported_length,
+                              export_size - exported_length ) );
     if( status != PSA_SUCCESS )
     {
         TEST_ASSERT( exported_length == 0 );