diff --git a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_handlers.cc b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_handlers.cc
index adcb4b0d6b9dc..cb8356ef0a89b 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_handlers.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_handlers.cc
@@ -1221,8 +1221,12 @@ error::Error GLES2DecoderPassthroughImpl::HandleVertexAttribIPointer(
   GLint size = static_cast<GLint>(c.size);
   GLenum type = static_cast<GLenum>(c.type);
   GLsizei stride = static_cast<GLsizei>(c.stride);
-  GLsizei offset = static_cast<GLsizei>(c.offset);
-  const void* ptr = reinterpret_cast<const void*>(offset);
+  // c.offset is a renderer-controlled uint32_t. Narrowing to GLsizei (int32)
+  // before reinterpret_cast<> sign-extends values >= 0x80000000 to a negative
+  // GLintptr on LP64, which ANGLE uses as a backward displacement into its
+  // shared BufferBlock pool. Zero-extend instead.
+  GLuint offset = static_cast<GLuint>(c.offset);
+  const void* ptr = reinterpret_cast<const void*>(static_cast<uintptr_t>(offset));
 
   return DoVertexAttribIPointer(index, size, type, stride, ptr);
 }
@@ -1237,8 +1241,12 @@ error::Error GLES2DecoderPassthroughImpl::HandleVertexAttribPointer(
   GLenum type = static_cast<GLenum>(c.type);
   GLboolean normalized = static_cast<GLenum>(c.normalized);
   GLsizei stride = static_cast<GLsizei>(c.stride);
-  GLsizei offset = static_cast<GLsizei>(c.offset);
-  const void* ptr = reinterpret_cast<const void*>(offset);
+  // c.offset is a renderer-controlled uint32_t. Narrowing to GLsizei (int32)
+  // before reinterpret_cast<> sign-extends values >= 0x80000000 to a negative
+  // GLintptr on LP64, which ANGLE uses as a backward displacement into its
+  // shared BufferBlock pool. Zero-extend instead.
+  GLuint offset = static_cast<GLuint>(c.offset);
+  const void* ptr = reinterpret_cast<const void*>(static_cast<uintptr_t>(offset));
 
   return DoVertexAttribPointer(index, size, type, normalized, stride, ptr);
 }