diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index 5f9e1ec..172c7f3 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -311,9 +311,15 @@
             Py_ssize_t i;
 
             p = va_arg(vargs, const char*);
-            i = strlen(p);
-            if (prec > 0 && i > prec)
-                i = prec;
+            if (prec <= 0) {
+                i = strlen(p);
+            }
+            else {
+                i = 0;
+                while (i < prec && p[i]) {
+                    i++;
+                }
+            }
             s = _PyBytesWriter_WriteBytes(&writer, s, p, i);
             if (s == NULL)
                 goto error;