diff --git a/src/gpu/glsl/GrGLSLVarying.h b/src/gpu/glsl/GrGLSLVarying.h
index 0da88a0..73048e7 100644
--- a/src/gpu/glsl/GrGLSLVarying.h
+++ b/src/gpu/glsl/GrGLSLVarying.h
@@ -16,6 +16,22 @@
 
 class GrGLSLProgramBuilder;
 
+#ifdef SK_DEBUG
+static bool is_matrix(GrSLType type) {
+    switch (type) {
+        case kFloat2x2_GrSLType:
+        case kFloat3x3_GrSLType:
+        case kFloat4x4_GrSLType:
+        case kHalf2x2_GrSLType:
+        case kHalf3x3_GrSLType:
+        case kHalf4x4_GrSLType:
+            return true;
+        default:
+            return false;
+    }
+}
+#endif
+
 class GrGLSLVarying {
 public:
     enum class Scope {
@@ -25,9 +41,16 @@
     };
 
     GrGLSLVarying() = default;
-    GrGLSLVarying(GrSLType type, Scope scope = Scope::kVertToFrag) : fType(type), fScope(scope) {}
+    GrGLSLVarying(GrSLType type, Scope scope = Scope::kVertToFrag)
+        : fType(type)
+        , fScope(scope) {
+        // Metal doesn't support varying matrices, so we disallow them everywhere for consistency
+        SkASSERT(!is_matrix(type));
+    }
 
     void reset(GrSLType type, Scope scope = Scope::kVertToFrag) {
+        // Metal doesn't support varying matrices, so we disallow them everywhere for consistency
+        SkASSERT(!is_matrix(type));
         *this = GrGLSLVarying();
         fType = type;
         fScope = scope;
diff --git a/src/sksl/SkSLIRGenerator.cpp b/src/sksl/SkSLIRGenerator.cpp
index 6f9fcd9..f9d4098 100644
--- a/src/sksl/SkSLIRGenerator.cpp
+++ b/src/sksl/SkSLIRGenerator.cpp
@@ -257,6 +257,11 @@
     if (!baseType) {
         return nullptr;
     }
+    if (fKind != Program::kFragmentProcessor_Kind &&
+        (decl.fModifiers.fFlags & Modifiers::kIn_Flag) &&
+        baseType->kind() == Type::Kind::kMatrix_Kind) {
+        fErrors.error(decl.fOffset, "'in' variables may not have matrix type");
+    }
     for (const auto& varDecl : decl.fVars) {
         if (decl.fModifiers.fLayout.fLocation == 0 && decl.fModifiers.fLayout.fIndex == 0 &&
             (decl.fModifiers.fFlags & Modifiers::kOut_Flag) && fKind == Program::kFragment_Kind &&