diff --git a/js/src/jit/arm/Assembler-arm.cpp b/js/src/jit/arm/Assembler-arm.cpp
index cbcefb65542a..f31b83396b41 100644
--- a/js/src/jit/arm/Assembler-arm.cpp
+++ b/js/src/jit/arm/Assembler-arm.cpp
@@ -3163,16 +3163,26 @@ Assembler::PatchDataWithValueCheck(CodeLocationLabel label, PatchedImmPtr newVal
 
     Register dest;
     Assembler::RelocStyle rs;
-    DebugOnly<const uint32_t*> val = GetPtr32Target(InstructionIterator(ptr), &dest, &rs);
-    MOZ_ASSERT(uint32_t((const uint32_t*)val) == uint32_t(expectedValue.value));
 
-    MacroAssembler::ma_mov_patch(Imm32(int32_t(newValue.value)), dest, Always, rs,
-                                 InstructionIterator(ptr));
+#ifdef DEBUG
+    {
+        InstructionIterator iter(ptr);
+        const uint32_t* val = GetPtr32Target(iter, &dest, &rs);
+        MOZ_ASSERT(uint32_t((const uint32_t*)val) == uint32_t(expectedValue.value));
+    }
+#endif
+
+    // Patch over actual instructions.
+    {
+        InstructionIterator iter(ptr);
+        MacroAssembler::ma_mov_patch(Imm32(int32_t(newValue.value)), dest, Always, rs, iter);
+    }
 
     // L_LDR won't cause any instructions to be updated.
     if (rs != L_LDR) {
-        AutoFlushICache::flush(uintptr_t(ptr), 4);
-        AutoFlushICache::flush(uintptr_t(ptr->next()), 4);
+        InstructionIterator iter(ptr);
+        AutoFlushICache::flush(uintptr_t(iter.cur()), 4);
+        AutoFlushICache::flush(uintptr_t(iter.next()), 4);
     }
 }
 
diff --git a/js/src/jit/arm/MacroAssembler-arm.cpp b/js/src/jit/arm/MacroAssembler-arm.cpp
index 274c4bb313d2..ad9341c9de00 100644
--- a/js/src/jit/arm/MacroAssembler-arm.cpp
+++ b/js/src/jit/arm/MacroAssembler-arm.cpp
@@ -353,11 +353,10 @@ void
 MacroAssemblerARM::ma_mov_patch(Imm32 imm32, Register dest, Assembler::Condition c,
                                 RelocStyle rs, Iter iter)
 {
+    // The current instruction must be an actual instruction,
+    // not automatically-inserted boilerplate.
     MOZ_ASSERT(iter.cur());
-
-    // Make sure the current instruction is not an artificial guard inserted
-    // by the assembler buffer.
-    iter.maybeSkipAutomaticInstructions();
+    MOZ_ASSERT(iter.cur() == iter.cur()->maybeSkipAutomaticInstructions());
 
     int32_t imm = imm32.value;
     switch(rs) {