diff --git a/ptrace/binding/func.py b/ptrace/binding/func.py
index ca70659..16a9fae 100644
--- a/ptrace/binding/func.py
+++ b/ptrace/binding/func.py
@@ -135,7 +135,7 @@
     HAS_CPTRACE = True
 except ImportError:
     HAS_CPTRACE = False
-    from ctypes import c_long, c_ulong
+    from ctypes import c_ulong
     from ptrace.ctypes_libc import libc
 
     # Load ptrace() function from the system C library
@@ -155,8 +155,8 @@ def ptrace(command, pid=0, arg1=0, arg2=0, check_errno=False):
             raise PtraceError(message, errno=errno, pid=pid)
     else:
         result = _ptrace(command, pid, arg1, arg2)
-        result_signed = c_long(result).value
-        if result_signed == -1:
+        result_unsigned = c_ulong(result).value
+        if result_unsigned == c_ulong(-1).value:
             errno = get_errno()
             # peek operations may returns -1 with errno=0:
             # it's not an error. For other operations, -1