diff --git a/src/text/gpu/SubRunContainer.cpp b/src/text/gpu/SubRunContainer.cpp
index bd5f827cc93..b486f57c782 100644
--- a/src/text/gpu/SubRunContainer.cpp
+++ b/src/text/gpu/SubRunContainer.cpp
@@ -937,6 +937,11 @@ public:
         auto vertexFiller = make_vertex_filler_from_buffer(buffer, alloc, &glyphVector.value(),
                                                            skglyph::kSDFT, kGlyphInsetting);
         if (!buffer.validate(vertexFiller.has_value())) { return nullptr; }
+        // SDFTSubRun must be kA8; the GP stride is computed assuming kA8 and a
+        // forged kARGB causes a stride mismatch in fillVertexData.
+        if (!buffer.validate(vertexFiller->maskFormat() == MaskFormat::kA8)) {
+            return nullptr;
+        }
 
         return alloc->makeUnique<SDFTSubRun>(useLCD,
                                              isAntiAliased,