diff --git a/src/core/SkRuntimeEffect.cpp b/src/core/SkRuntimeEffect.cpp
index aa84051..4b12857 100644
--- a/src/core/SkRuntimeEffect.cpp
+++ b/src/core/SkRuntimeEffect.cpp
@@ -317,8 +317,14 @@
                 c.type  = child_type(varType);
                 c.index = children.size();
                 children.push_back(c);
-                sampleUsages.push_back(SkSL::Analysis::GetSampleUsage(
-                        *program, var, sampleCoordsUsage.fWrite != 0, &elidedSampleCoords));
+                auto usage = SkSL::Analysis::GetSampleUsage(
+                        *program, var, sampleCoordsUsage.fWrite != 0, &elidedSampleCoords);
+                // If the child is never sampled, we pretend that it's actually in PassThrough mode.
+                // Otherwise, the GP code for collecting transforms and emitting transform code gets
+                // very confused, leading to asserts and bad (backend) shaders. There's an implicit
+                // assumption that every FP is used by its parent. (skbug.com/12429)
+                sampleUsages.push_back(usage.isSampled() ? usage
+                                                         : SkSL::SampleUsage::PassThrough());
             }
             // 'uniform' variables
             else if (var.modifiers().fFlags & SkSL::Modifiers::kUniform_Flag) {
diff --git a/src/gpu/GrFragmentProcessor.cpp b/src/gpu/GrFragmentProcessor.cpp
index 0d5f537..390105d 100644
--- a/src/gpu/GrFragmentProcessor.cpp
+++ b/src/gpu/GrFragmentProcessor.cpp
@@ -142,6 +142,8 @@
 
 void GrFragmentProcessor::registerChild(std::unique_ptr<GrFragmentProcessor> child,
                                         SkSL::SampleUsage sampleUsage) {
+    SkASSERT(sampleUsage.isSampled());
+
     if (!child) {
         fChildProcessors.push_back(nullptr);
         return;
diff --git a/tests/SkRuntimeEffectTest.cpp b/tests/SkRuntimeEffectTest.cpp
index bf90812..428af17 100644
--- a/tests/SkRuntimeEffectTest.cpp
+++ b/tests/SkRuntimeEffectTest.cpp
@@ -498,6 +498,13 @@
     effect.child("child") = rgbwShader;
     effect.test({0xFF0000FF, 0xFFFF0000, 0xFF00FF00, 0xFFFFFFFF});
 
+    // Bind an image shader, but don't use it - ensure that we don't assert or generate bad shaders.
+    // (skbug.com/12429)
+    effect.build("uniform shader child;"
+                 "half4 main(float2 p) { return half4(0, 1, 0, 1); }");
+    effect.child("child") = rgbwShader;
+    effect.test(0xFF00FF00);
+
     //
     // Helper functions
     //