diff --git a/src/providers/postgres/qgspostgresprovider.cpp b/src/providers/postgres/qgspostgresprovider.cpp
index c8d8658ff43..18eae093a3d 100644
--- a/src/providers/postgres/qgspostgresprovider.cpp
+++ b/src/providers/postgres/qgspostgresprovider.cpp
@@ -52,6 +52,26 @@ inline int32_t FID2PKINT( int64_t x )
   return QgsPostgresUtils::fid_to_int32pk( x );
 }
 
+QgsPostgresPrimaryKeyType
+QgsPostgresProvider::pkType( const QgsField& f ) const
+{
+  switch ( f.type() )
+  {
+    case QVariant::LongLong:
+      // unless we can guarantee all values are unsigned
+      // (in which case we could use pktUint64)
+      // we'll have to use a Map type.
+      // See http://hub.qgis.org/issues/14262
+      return pktFidMap; // pktUint64
+
+    case QVariant::Int:
+      return pktInt;
+
+    default:
+      return pktFidMap;
+  }
+}
+
 
 
 QgsPostgresProvider::QgsPostgresProvider( QString const & uri )
@@ -1312,7 +1332,7 @@ bool QgsPostgresProvider::determinePrimaryKey()
       QString primaryKey;
       QString delim = "";
 
-      mPrimaryKeyType = pktFidMap; // int by default, will downgrade if needed
+      mPrimaryKeyType = pktFidMap; // map by default, will downgrade if needed
       for ( int i = 0; i < res.PQntuples(); i++ )
       {
         QString name = res.PQgetvalue( i, 0 );
@@ -1333,22 +1353,8 @@ bool QgsPostgresProvider::determinePrimaryKey()
         }
         const QgsField& fld = mAttributeFields.at( idx );
 
-        if ( i )
-        {
-          mPrimaryKeyType = pktFidMap; // multi-field
-        }
-        else if ( fld.type() == QVariant::LongLong )
-        {
-          // unless we can guarantee all values are unsigned
-          // (in which case we could use pktUint64)
-          // we'll have to use a Map type.
-          // See http://hub.qgis.org/issues/14262
-          mPrimaryKeyType = pktFidMap; // pktUint64
-        }
-        else if ( fld.type() == QVariant::Int )
-        {
-          mPrimaryKeyType = pktInt;
-        }
+        // Always use pktFidMap for multi-field keys
+        mPrimaryKeyType = i ? pktFidMap : pkType( fld );
 
         mPrimaryKeyAttrs << idx;
       }
@@ -1444,14 +1450,7 @@ void QgsPostgresProvider::determinePrimaryKeyFromUriKeyColumn()
         if ( mPrimaryKeyAttrs.size() == 1 )
         {
           const QgsField& fld = mAttributeFields.at( 0 );
-          if ( fld.type() == QVariant::LongLong )
-          {
-            mPrimaryKeyType = pktUint64; // 64bit integer
-          }
-          else if ( fld.type() == QVariant::Int )
-          {
-            mPrimaryKeyType = pktInt; // account for signed
-          }
+          mPrimaryKeyType = pkType( fld );
         }
       }
       else
diff --git a/src/providers/postgres/qgspostgresprovider.h b/src/providers/postgres/qgspostgresprovider.h
index ca5a5bc1bdb..46592522cdf 100644
--- a/src/providers/postgres/qgspostgresprovider.h
+++ b/src/providers/postgres/qgspostgresprovider.h
@@ -352,6 +352,17 @@ class QgsPostgresProvider : public QgsVectorDataProvider
      */
     bool parseDomainCheckConstraint( QStringList& enumValues, const QString& attributeName ) const;
 
+    /** Return the type of primary key for a PK field
+     *
+     * @param fld the field to determine PK type of
+     * @return the PrimaryKeyType
+     *
+     * @note that this only makes sense for single-field primary keys,
+     *       whereas multi-field keys always need the pktFidMap
+     *       primary key type.
+     */
+    QgsPostgresPrimaryKeyType pkType( const QgsField& fld ) const;
+
     QgsFields mAttributeFields;
     QString mDataComment;
 
diff --git a/tests/src/python/test_provider_postgres.py b/tests/src/python/test_provider_postgres.py
index 6b170c0ed58..832c3a466f8 100644
--- a/tests/src/python/test_provider_postgres.py
+++ b/tests/src/python/test_provider_postgres.py
@@ -171,9 +171,7 @@ class TestPyQgsPostgresProvider(unittest.TestCase, ProviderTestCase):
             self.assertEqual(count, 1)
         test_query_attribute(self.dbconn, '(SELECT -1::int4 i, NULL::geometry(Point) g)', 'i', -1, 4294967295)
         test_query_attribute(self.dbconn, '(SELECT -2::int2 i, NULL::geometry(Point) g)', 'i', -2, 4294967294)
-        # Unfortunately negative int8 identifiers are still not supported at this moment
-        # TODO: fix expected fidval to be positive !
-        test_query_attribute(self.dbconn, '(SELECT -3::int8 i, NULL::geometry(Point) g)', 'i', -3, -3)
+        test_query_attribute(self.dbconn, '(SELECT -3::int8 i, NULL::geometry(Point) g)', 'i', -3, 1)
 
     def testPktIntInsert(self):
         vl = QgsVectorLayer('{} table="qgis_test"."{}" key="pk" sql='.format(self.dbconn, 'bikes_view'), "bikes_view", "postgres")