diff --git a/psycopg/cursor_type.c b/psycopg/cursor_type.c
index 159b9de8..b9295bcc 100644
--- a/psycopg/cursor_type.c
+++ b/psycopg/cursor_type.c
@@ -78,8 +78,8 @@ psyco_curs_close(cursorObject *self, PyObject *args)
 static int
 _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
 {
-    PyObject *key, *value, *n, *item;
-    char *d, *c;
+    PyObject *key, *value, *n;
+    const char *d, *c;
     Py_ssize_t index = 0;
     int force = 0, kind = 0;
 
@@ -90,19 +90,26 @@ _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
     c = Bytes_AsString(fmt);
 
     while(*c) {
-        /* handle plain percent symbol in format string */
-        if (c[0] == '%' && c[1] == '%') {
-            c+=2; force = 1;
+        if (*c++ != '%') {
+            /* a regular character */
+            continue;
         }
 
+        switch (*c) {
+
+        /* handle plain percent symbol in format string */
+        case '%':
+            ++c;
+            force = 1;
+            break;
+
         /* if we find '%(' then this is a dictionary, we:
            1/ find the matching ')' and extract the key name
            2/ locate the value in the dictionary (or return an error)
            3/ mogrify the value into something usefull (quoting)...
            4/ ...and add it to the new dictionary to be used as argument
         */
-        else if (c[0] == '%' && c[1] == '(') {
-
+        case '(':
             /* check if some crazy guy mixed formats */
             if (kind == 2) {
                 Py_XDECREF(n);
@@ -113,10 +120,10 @@ _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
             kind = 1;
 
             /* let's have d point the end of the argument */
-            for (d = c + 2; *d && *d != ')'; d++);
+            for (d = c + 1; *d && *d != ')' && *d != '%'; d++);
 
             if (*d == ')') {
-                key = Text_FromUTF8AndSize(c+2, (Py_ssize_t) (d-c-2));
+                key = Text_FromUTF8AndSize(c+1, (Py_ssize_t) (d-c-1));
                 value = PyObject_GetItem(var, key);
                 /* key has refcnt 1, value the original value + 1 */
 
@@ -135,11 +142,9 @@ _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
                     n = PyDict_New();
                 }
 
-                if ((item = PyObject_GetItem(n, key)) == NULL) {
+                if (0 == PyDict_Contains(n, key)) {
                     PyObject *t = NULL;
 
-                    PyErr_Clear();
-
                     /* None is always converted to NULL; this is an
                        optimization over the adapting code and can go away in
                        the future if somebody finds a None adapter useful. */
@@ -148,13 +153,6 @@ _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
                         t = psyco_null;
                         PyDict_SetItem(n, key, t);
                         /* t is a new object, refcnt = 1, key is at 2 */
-
-                        /* if the value is None we need to substitute the
-                           formatting char with 's' (FIXME: this should not be
-                           necessary if we drop support for formats other than
-                           %s!) */
-                        while (*d && !isalpha(*d)) d++;
-                        if (*d) *d = 's';
                     }
                     else {
                         t = microprotocol_getquoted(value, conn);
@@ -176,20 +174,21 @@ _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
                        if it was added to the dictionary directly; good */
                     Py_XDECREF(value);
                 }
-                else {
-                    /* we have an item with one extra refcnt here, zap! */
-                    Py_DECREF(item);
-                }
                 Py_DECREF(key); /* key has the original refcnt now */
                 Dprintf("_mogrify: after value refcnt: "
-                    FORMAT_CODE_PY_SSIZE_T,
-                    Py_REFCNT(value)
-                  );
+                    FORMAT_CODE_PY_SSIZE_T, Py_REFCNT(value));
             }
-            c = d;
-        }
+            else {
+                /* we found %( but not a ) */
+                Py_XDECREF(n);
+                psyco_set_error(ProgrammingError, (PyObject*)conn,
+                   "incomplete placeholder: '%(' without ')'", NULL, NULL);
+                return -1;
+            }
+            c = d + 1;  /* after the ) */
+            break;
 
-        else if (c[0] == '%' && c[1] != '(') {
+        default:
             /* this is a format that expects a tuple; it is much easier,
                because we don't need to check the old/new dictionary for
                keys */
@@ -218,13 +217,9 @@ _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
             }
 
             /* let's have d point just after the '%' */
-            d = c+1;
-
             if (value == Py_None) {
                 Py_INCREF(psyco_null);
                 PyTuple_SET_ITEM(n, index, psyco_null);
-                while (*d && !isalpha(*d)) d++;
-                if (*d) *d = 's';
                 Py_DECREF(value);
             }
             else {
@@ -240,12 +235,8 @@ _mogrify(PyObject *var, PyObject *fmt, connectionObject *conn, PyObject **new)
                     return -1;
                 }
             }
-            c = d;
             index += 1;
         }
-        else {
-            c++;
-        }
     }
 
     if (force && n == NULL)
diff --git a/tests/test_cursor.py b/tests/test_cursor.py
index 65050c81..b8a8b662 100755
--- a/tests/test_cursor.py
+++ b/tests/test_cursor.py
@@ -91,6 +91,17 @@ class CursorTests(unittest.TestCase):
         self.assertEqual(b('SELECT 10.3;'),
             cur.mogrify("SELECT %s;", (Decimal("10.3"),)))
 
+    def test_bad_placeholder(self):
+        cur = self.conn.cursor()
+        self.assertRaises(psycopg2.ProgrammingError,
+            cur.mogrify, "select %(foo", {})
+        self.assertRaises(psycopg2.ProgrammingError,
+            cur.mogrify, "select %(foo", {'foo': 1})
+        self.assertRaises(psycopg2.ProgrammingError,
+            cur.mogrify, "select %(foo, %(bar)", {'foo': 1})
+        self.assertRaises(psycopg2.ProgrammingError,
+            cur.mogrify, "select %(foo, %(bar)", {'foo': 1, 'bar': 2})
+
     def test_cast(self):
         curs = self.conn.cursor()