diff -r 89e2201142f9 Lib/sqlite3/test/types.py
--- a/Lib/sqlite3/test/types.py	Fri Nov 18 10:42:10 2016 -0800
+++ b/Lib/sqlite3/test/types.py	Fri Nov 18 23:39:21 2016 +0100
@@ -74,6 +74,24 @@
         row = self.cur.fetchone()
         self.assertEqual(row[0], sample)
 
+    def CheckErrorInAdapt(self):
+        class Point:
+            def __init__(self, x, y):
+                self.x, self.y = x, y
+
+        def adapt_point(point):
+            assert False, 'Problem in adapter'
+
+        sqlite.register_adapter(Point, adapt_point)
+
+        p = Point(4.0, -3.2)
+        try:
+            self.cur.execute("select ?", (p,))
+        except AssertionError as ex:
+            self.assertEqual(str(ex), 'Problem in adapter')
+        else:
+            assert False, 'Should have raised'
+
     def CheckUnicodeExecute(self):
         self.cur.execute("select 'Österreich'")
         row = self.cur.fetchone()
diff -r 89e2201142f9 Modules/_sqlite/microprotocols.c
--- a/Modules/_sqlite/microprotocols.c	Fri Nov 18 10:42:10 2016 -0800
+++ b/Modules/_sqlite/microprotocols.c	Fri Nov 18 23:39:21 2016 +0100
@@ -49,7 +49,9 @@
 }
 
 
-/* pysqlite_microprotocols_add - add a reverse type-caster to the dictionary */
+/* pysqlite_microprotocols_add - add a reverse type-caster to the dictionary
+Returns NULL without an exception in the case there's legitimately
+nothing to do. */
 
 int
 pysqlite_microprotocols_add(PyTypeObject *type, PyObject *proto, PyObject *cast)
@@ -127,9 +129,6 @@
             return NULL;
         }
     }
-
-    /* else set the right exception and return NULL */
-    PyErr_SetString(pysqlite_ProgrammingError, "can't adapt");
     return NULL;
 }
 
@@ -140,7 +139,17 @@
 {
     PyObject *obj, *alt = NULL;
     PyObject *proto = (PyObject*)&pysqlite_PrepareProtocolType;
+    PyObject *adapted;
 
-    if (!PyArg_ParseTuple(args, "O|OO", &obj, &proto, &alt)) return NULL;
-    return pysqlite_microprotocols_adapt(obj, proto, alt);
+    if (!PyArg_ParseTuple(args, "O|OO", &obj, &proto, &alt))
+        return NULL;
+    adapted = pysqlite_microprotocols_adapt(obj, proto, alt);
+    if (adapted != NULL) {
+        return adapted;
+    } else {
+        Py_DECREF(adapted);
+        if (!PyErr_Occurred())
+            PyErr_SetString(pysqlite_ProgrammingError, "can't adapt");
+        return NULL;
+    }
 }
diff -r 89e2201142f9 Modules/_sqlite/statement.c
--- a/Modules/_sqlite/statement.c	Fri Nov 18 10:42:10 2016 -0800
+++ b/Modules/_sqlite/statement.c	Fri Nov 18 23:39:21 2016 +0100
@@ -252,7 +252,8 @@
                 if (adapted) {
                     Py_DECREF(current_param);
                 } else {
-                    PyErr_Clear();
+                    if (PyErr_Occurred())
+                        return;
                     adapted = current_param;
                 }
             }
@@ -297,7 +298,8 @@
                 if (adapted) {
                     Py_DECREF(current_param);
                 } else {
-                    PyErr_Clear();
+                    if (PyErr_Occurred())
+                        return;
                     adapted = current_param;
                 }
             }