diff --git a/src/core/qgsclipper.h b/src/core/qgsclipper.h
index d3a7d7a820e..4a0f7522449 100644
--- a/src/core/qgsclipper.h
+++ b/src/core/qgsclipper.h
@@ -566,9 +566,9 @@ inline bool QgsClipper::inside( double x, double y, double z, Boundary boundary,
     case YMin: // y > MIN_Y is inside
       return ( y > boundaryValue );
     case ZMax: // z < MAX_Z is inside
-      return ( z < boundaryValue );
+      return z < boundaryValue || std::isnan( z ) ;
     case ZMin: // z > MIN_Z is inside
-      return ( z > boundaryValue );
+      return z > boundaryValue || std::isnan( z );
   }
   return false;
 }
diff --git a/tests/src/core/testqgsclipper.cpp b/tests/src/core/testqgsclipper.cpp
index 31b5a0d0c23..9f752528923 100644
--- a/tests/src/core/testqgsclipper.cpp
+++ b/tests/src/core/testqgsclipper.cpp
@@ -42,6 +42,7 @@ class TestQgsClipper: public QgsTest
     void basicWithZ();
     void basicWithZInf();
     void epsg4978LineRendering();
+    void clipGeometryWithNaNZValues();
 
   private:
 
@@ -202,6 +203,50 @@ void TestQgsClipper::epsg4978LineRendering()
   QVERIFY( render2dCheck( "4978_2D_line_rendering", layerLines.get(), QgsRectangle( -2.5e7, -2.5e7, 2.5e7, 2.5e7 ) ) );
 }
 
+void TestQgsClipper::clipGeometryWithNaNZValues()
+{
+  // nan z values should not result in clipping
+  QgsGeometry geom = QgsGeometry::fromWkt( QStringLiteral( "PolygonZ ((704425.82266869 7060014.33574043 19.51, 704439.59844559 7060023.73007711 19.69, 704441.6748229 7060020.65665367 19.63, 704428.333268 7060011.65915509 19.42, 704428.15434668 7060011.92446088 0, 704441.23037799 7060020.74289127 0, 704439.51320673 7060023.28462315 0, 704426.00295955 7060014.07136342 0, 704425.82266869 7060014.33574043 19.51))" ) );
+  QgsLineString *exteriorRing = qgsgeometry_cast< QgsLineString * >( qgsgeometry_cast< QgsPolygon *>( geom.get() )->exteriorRing() );
+  exteriorRing->setZAt( 4, std::numeric_limits<double>::quiet_NaN() );
+  exteriorRing->setZAt( 5, std::numeric_limits<double>::quiet_NaN() );
+  exteriorRing->setZAt( 6, std::numeric_limits<double>::quiet_NaN() );
+  exteriorRing->setZAt( 7, std::numeric_limits<double>::quiet_NaN() );
+
+  QPolygonF polygon;
+  polygon << QPointF( 10.4, 20.5 ) << QPointF( 20.2, 30.2 );
+
+  QVector< double > pointsX = exteriorRing->xVector();
+  QVector< double > pointsY = exteriorRing->yVector();
+  QVector< double > pointsZ = exteriorRing->zVector();
+  QCOMPARE( pointsX.size(), 9 );
+
+  // 2d trim
+  QgsRectangle clipRect( 704430.30, 7060007.72, 704456.51, 7060029.03 );
+  QgsClipper::trimPolygon( pointsX, pointsY, pointsZ, clipRect );
+  // one vertex should be clipped
+  QCOMPARE( pointsX.size(), 8 );
+  QCOMPARE( pointsY.size(), 8 );
+  QCOMPARE( pointsZ.size(), 8 );
+  QGSCOMPARENEAR( pointsX.at( 0 ), 704430.30, 0.01 );
+  QGSCOMPARENEAR( pointsY.at( 0 ), 7060017.389, 0.01 );
+  QGSCOMPARENEAR( pointsZ.at( 0 ), 19.568, 0.01 );
+
+  // 3d trim, clip box contains whole geometry
+  pointsX = exteriorRing->xVector();
+  pointsY = exteriorRing->yVector();
+  pointsZ = exteriorRing->zVector();
+  QgsBox3d clipRect3D( 704430.30, 7060007.72, 0, 704456.51, 7060029.03, 100 );
+  QgsClipper::trimPolygon( pointsX, pointsY, pointsZ, clipRect3D );
+  // still only one vertex should be clipped, the nan z values must remain
+  QCOMPARE( pointsX.size(), 8 );
+  QCOMPARE( pointsY.size(), 8 );
+  QCOMPARE( pointsZ.size(), 8 );
+  QGSCOMPARENEAR( pointsX.at( 0 ), 704430.30, 0.01 );
+  QGSCOMPARENEAR( pointsY.at( 0 ), 7060017.389, 0.01 );
+  QGSCOMPARENEAR( pointsZ.at( 0 ), 19.568, 0.01 );
+}
+
 bool TestQgsClipper::render2dCheck( const QString &testName, QgsVectorLayer *layer, QgsRectangle extent )
 {
   const QString myTmpDir = QDir::tempPath() + '/';
diff --git a/tests/src/python/test_qgsgeometrygeneratorsymbollayer.py b/tests/src/python/test_qgsgeometrygeneratorsymbollayer.py
index c82ce0b3ef6..1a46b00814a 100644
--- a/tests/src/python/test_qgsgeometrygeneratorsymbollayer.py
+++ b/tests/src/python/test_qgsgeometrygeneratorsymbollayer.py
@@ -483,6 +483,40 @@ class TestQgsGeometryGeneratorSymbolLayerV2(unittest.TestCase):
         self.report += renderchecker.report()
         self.assertTrue(res)
 
+    def test_clipped_results_with_z(self):
+        """
+        See https://github.com/qgis/QGIS/issues/51796
+        """
+        lines = QgsVectorLayer('LineString?crs=epsg:2154', 'Lines', 'memory')
+        self.assertTrue(lines.isValid())
+        f = QgsFeature()
+        f.setGeometry(QgsGeometry.fromWkt('LineStringZ (704425.82266868802253157 7060014.33574043028056622 19.51000000000000156, 704439.59844558802433312 7060023.7300771102309227 19.69000000000000128, 704441.67482289997860789 7060020.65665366966277361 19.62999999999999901, 704428.333267995971255 7060011.65915509033948183 19.42000000000000171)'))
+        lines.dataProvider().addFeature(f)
+
+        subsymbol = QgsFillSymbol.createSimple({'color': '#0000ff', 'line_style': 'no'})
+
+        parent_generator = QgsGeometryGeneratorSymbolLayer.create(
+            {'geometryModifier': 'single_sided_buffer($geometry,-0.32, 1, 2)'})
+        parent_generator.setSymbolType(QgsSymbol.Fill)
+
+        parent_generator.setSubSymbol(subsymbol)
+
+        geom_symbol = QgsLineSymbol()
+        geom_symbol.changeSymbolLayer(0, parent_generator)
+        lines.renderer().setSymbol(geom_symbol)
+
+        mapsettings = QgsMapSettings(self.mapsettings)
+        mapsettings.setDestinationCrs(lines.crs())
+        mapsettings.setExtent(QgsRectangle(704433.77, 7060006.64, 704454.78, 7060027.95))
+        mapsettings.setLayers([lines])
+
+        renderchecker = QgsMultiRenderChecker()
+        renderchecker.setMapSettings(mapsettings)
+        renderchecker.setControlName('expected_geometrygenerator_z_clipping')
+        res = renderchecker.runTest('geometrygenerator_z_clipping')
+        self.report += renderchecker.report()
+        self.assertTrue(res)
+
     def imageCheck(self, name, reference_image, image):
         self.report += f"<h2>Render {name}</h2>\n"
         temp_dir = QDir.tempPath() + '/'
diff --git a/tests/testdata/control_images/expected_geometrygenerator_z_clipping/expected_geometrygenerator_z_clipping.png b/tests/testdata/control_images/expected_geometrygenerator_z_clipping/expected_geometrygenerator_z_clipping.png
new file mode 100644
index 00000000000..d0ba77a26c2
Binary files /dev/null and b/tests/testdata/control_images/expected_geometrygenerator_z_clipping/expected_geometrygenerator_z_clipping.png differ