diff --git a/win32ss/user/winsrv/consrv/alias.c b/win32ss/user/winsrv/consrv/alias.c
index 3acae7a4ef5..3d7a533d5a0 100644
--- a/win32ss/user/winsrv/consrv/alias.c
+++ b/win32ss/user/winsrv/consrv/alias.c
@@ -70,6 +70,14 @@ ConvertInputUnicodeToAnsi(PCONSOLE Console,
 {
     ASSERT(Source && Target && TargetLength);
 
+    /*
+     * From MSDN:
+     * "The lpMultiByteStr and lpWideCharStr pointers must not be the same.
+     *  If they are the same, the function fails, and GetLastError returns
+     *  ERROR_INVALID_PARAMETER."
+     */
+    ASSERT((ULONG_PTR)Source != (ULONG_PTR)Target);
+
     /* Use the console input CP for the conversion */
     // *TargetLength = WideCharToMultiByte(Console->InputCodePage, 0,
                                         // Source, SourceLength,
diff --git a/win32ss/user/winsrv/consrv/condrv/coninput.c b/win32ss/user/winsrv/consrv/condrv/coninput.c
index c7b5e27b765..83428a97e85 100644
--- a/win32ss/user/winsrv/consrv/condrv/coninput.c
+++ b/win32ss/user/winsrv/consrv/condrv/coninput.c
@@ -16,10 +16,18 @@
 
 /* GLOBALS ********************************************************************/
 
+/*
+ * From MSDN:
+ * "The lpMultiByteStr and lpWideCharStr pointers must not be the same.
+ *  If they are the same, the function fails, and GetLastError returns
+ *  ERROR_INVALID_PARAMETER."
+ */
 #define ConsoleInputUnicodeCharToAnsiChar(Console, dChar, sWChar) \
+    ASSERT((ULONG_PTR)dChar != (ULONG_PTR)sWChar); \
     WideCharToMultiByte((Console)->InputCodePage, 0, (sWChar), 1, (dChar), 1, NULL, NULL)
 
 #define ConsoleInputAnsiCharToUnicodeChar(Console, dWChar, sChar) \
+    ASSERT((ULONG_PTR)dWChar != (ULONG_PTR)sChar); \
     MultiByteToWideChar((Console)->InputCodePage, 0, (sChar), 1, (dWChar), 1)
 
 typedef struct ConsoleInput_t
diff --git a/win32ss/user/winsrv/consrv/include/conio.h b/win32ss/user/winsrv/consrv/include/conio.h
index f4985e87c6f..5e8399e8461 100644
--- a/win32ss/user/winsrv/consrv/include/conio.h
+++ b/win32ss/user/winsrv/consrv/include/conio.h
@@ -352,10 +352,18 @@ do {    \
 #define ConioRectWidth(Rect) \
     (((Rect)->Left) > ((Rect)->Right) ? 0 : ((Rect)->Right) - ((Rect)->Left) + 1)
 
+/*
+ * From MSDN:
+ * "The lpMultiByteStr and lpWideCharStr pointers must not be the same.
+ *  If they are the same, the function fails, and GetLastError returns
+ *  ERROR_INVALID_PARAMETER."
+ */
 #define ConsoleUnicodeCharToAnsiChar(Console, dChar, sWChar) \
+    ASSERT((ULONG_PTR)dChar != (ULONG_PTR)sWChar); \
     WideCharToMultiByte((Console)->OutputCodePage, 0, (sWChar), 1, (dChar), 1, NULL, NULL)
 
 #define ConsoleAnsiCharToUnicodeChar(Console, dWChar, sChar) \
+    ASSERT((ULONG_PTR)dWChar != (ULONG_PTR)sChar); \
     MultiByteToWideChar((Console)->OutputCodePage, 0, (sChar), 1, (dWChar), 1)
 
 PCHAR_INFO ConioCoordToPointer(PTEXTMODE_SCREEN_BUFFER Buff, ULONG X, ULONG Y);
diff --git a/win32ss/user/winsrv/consrv/include/conio_winsrv.h b/win32ss/user/winsrv/consrv/include/conio_winsrv.h
index b4e08483e9f..957a8ee26d3 100644
--- a/win32ss/user/winsrv/consrv/include/conio_winsrv.h
+++ b/win32ss/user/winsrv/consrv/include/conio_winsrv.h
@@ -161,10 +161,18 @@ do {    \
 #define ConioRectWidth(Rect) \
     (((Rect)->Left) > ((Rect)->Right) ? 0 : ((Rect)->Right) - ((Rect)->Left) + 1)
 
+/*
+ * From MSDN:
+ * "The lpMultiByteStr and lpWideCharStr pointers must not be the same.
+ *  If they are the same, the function fails, and GetLastError returns
+ *  ERROR_INVALID_PARAMETER."
+ */
 #define ConsoleUnicodeCharToAnsiChar(Console, dChar, sWChar) \
+    ASSERT((ULONG_PTR)dChar != (ULONG_PTR)sWChar); \
     WideCharToMultiByte((Console)->OutputCodePage, 0, (sWChar), 1, (dChar), 1, NULL, NULL)
 
 #define ConsoleAnsiCharToUnicodeChar(Console, dWChar, sChar) \
+    ASSERT((ULONG_PTR)dWChar != (ULONG_PTR)sChar); \
     MultiByteToWideChar((Console)->OutputCodePage, 0, (sChar), 1, (dWChar), 1)
 
 PCHAR_INFO ConioCoordToPointer(PTEXTMODE_SCREEN_BUFFER Buff, ULONG X, ULONG Y);