diff --git a/X86Disassembler/X86/ModRMDecoder.cs b/X86Disassembler/X86/ModRMDecoder.cs
index 0a4f6d4..3f6ee08 100644
--- a/X86Disassembler/X86/ModRMDecoder.cs
+++ b/X86Disassembler/X86/ModRMDecoder.cs
@@ -403,7 +403,9 @@ public class ModRMDecoder
         else // Memory operand
         {
             // For memory operands, we need to map the RegisterIndex8 to RegisterIndex for base registers
-            RegisterIndex rmRegIndex = MapRegister8ToBaseRegister(rm);
+            // The rmIndex is the raw value from the ModR/M byte, not the mapped RegisterIndex8
+            // This is important because we need to check if it's 4 (ESP) for SIB byte
+            RegisterIndex rmRegIndex = MapModRMToRegisterIndex(rmIndex);
             
             // Use the DecodeModRM8 method to get an 8-bit memory operand
             operand = DecodeModRM8(mod, rmRegIndex);
diff --git a/X86DisassemblerTests/InstructionTests/OrRm8R8HandlerTests.cs b/X86DisassemblerTests/InstructionTests/OrRm8R8HandlerTests.cs
index 3712408..25e0a27 100644
--- a/X86DisassemblerTests/InstructionTests/OrRm8R8HandlerTests.cs
+++ b/X86DisassemblerTests/InstructionTests/OrRm8R8HandlerTests.cs
@@ -43,10 +43,10 @@ public class OrRm8R8HandlerTests
         
         // Check the second operand (AL)
         var alOperand = instruction.StructuredOperands[1];
-        Assert.IsType<RegisterOperand>(alOperand);
-        var registerOperand = (RegisterOperand)alOperand;
-        Assert.Equal(RegisterIndex.A, registerOperand.Register);
-        Assert.Equal(8, registerOperand.Size); // Validate that it's an 8-bit register (AL)
+        Assert.IsType<Register8Operand>(alOperand);
+        var register8Operand = (Register8Operand)alOperand;
+        Assert.Equal(RegisterIndex8.AL, register8Operand.Register);
+        Assert.Equal(8, register8Operand.Size); // Validate that it's an 8-bit register (AL)
     }
     
     /// <summary>