diff --git a/Lib/test/test_b1.py b/Lib/test/test_b1.py
index 8d47abc7370..554e350fda2 100644
--- a/Lib/test/test_b1.py
+++ b/Lib/test/test_b1.py
@@ -124,16 +124,29 @@ if complex(0j, 3.14) != 3.14j: raise TestFailed, 'complex(0j, 3.14)'
 if complex(0.0, 3.14) != 3.14j: raise TestFailed, 'complex(0.0, 3.14)'
 if complex("1") != 1+0j: raise TestFailed, 'complex("1")'
 if complex("1j") != 1j: raise TestFailed, 'complex("1j")'
+
 try: complex("1", "1")
 except TypeError: pass
 else: raise TestFailed, 'complex("1", "1")'
+
 try: complex(1, "1")
 except TypeError: pass
 else: raise TestFailed, 'complex(1, "1")'
+
 if complex("  3.14+J  ") != 3.14+1j:  raise TestFailed, 'complex("  3.14+J  )"'
 if have_unicode:
     if complex(unicode("  3.14+J  ")) != 3.14+1j:
         raise TestFailed, 'complex(u"  3.14+J  )"'
+
+# SF bug 543840:  complex(string) accepts strings with \0
+# Fixed in 2.3.
+try:
+    complex('1+1j\0j')
+except ValueError:
+    pass
+else:
+    raise TestFailed("complex('1+1j\0j') should have raised ValueError")
+
 class Z:
     def __complex__(self): return 3.14j
 z = Z()
diff --git a/Objects/complexobject.c b/Objects/complexobject.c
index 3dbaea364b7..c074aee7fb9 100644
--- a/Objects/complexobject.c
+++ b/Objects/complexobject.c
@@ -787,7 +787,7 @@ complex_subtype_from_string(PyTypeObject *type, PyObject *v)
 
 		}  /* end of switch  */
 
-	} while (*s!='\0' && !sw_error);
+	} while (s - start < len && !sw_error);
 
 	if (sw_error) {
 		PyErr_SetString(PyExc_ValueError,