C/C++ Inline asm improper operand type

I have the following code, that is supposed to XOR a block of memory:

void XorBlock(DWORD dwStartAddress, DWORD dwSize, DWORD dwsKey)
{
DWORD dwKey;
__asm
{
    push eax
    push ecx
    mov ecx, dwStartAddress          // Move Start Address to ECX
    add ecx, dwSize                  // Add the size of the function to ECX
    mov eax, dwStartAddress          // Copy the Start Address to EAX

    crypt_loop:                         // Start of the loop
        xor byte ptr ds:[eax], dwKey     // XOR The current byte with 0x4D
        inc eax                         // Increment EAX with dwStartAddress++
        cmp eax,ecx                     // Check if every byte is XORed
    jl crypt_loop;                      // Else jump back to the start label

    pop ecx // pop ECX from stack
    pop eax // pop EAX from stack
}
}

However, the argument dwKey gives me an error. The code works perfectly if for example the dwKey is replaced by 0x5D.