diff --git a/win32ss/gdi/ntgdi/coord.c b/win32ss/gdi/ntgdi/coord.c
index 58d236581b..0f7c1ed191 100644
--- a/win32ss/gdi/ntgdi/coord.c
+++ b/win32ss/gdi/ntgdi/coord.c
@@ -213,7 +213,7 @@ DC_vUpdateDeviceToWorld(PDC pdc)
     XFORMOBJ_vInit(&xoDeviceToWorld, &pdc->pdcattr->mxDeviceToWorld);
     if (XFORMOBJ_iInverse(&xoDeviceToWorld, &xoWorldToDevice) == DDI_ERROR)
     {
-        // FIXME: do we need to reset anything?
+        MX_Set0(&pdc->pdcattr->mxDeviceToWorld);
         return;
     }
 
@@ -411,6 +411,13 @@ NtGdiTransformPoints(
     switch (iMode)
     {
         case GdiDpToLp:
+            if (pdc->pdcattr->iMapMode == MM_ISOTROPIC &&
+                (pdc->pdcattr->szlViewportExt.cx * pdc->pdcattr->szlWindowExt.cy !=
+                 pdc->pdcattr->szlWindowExt.cx * pdc->pdcattr->szlViewportExt.cy))
+            {
+                ret = FALSE;
+                goto leave;
+            }
             DC_vXformDeviceToWorld(pdc, Count, Points, Points);
             break;
 
@@ -595,7 +602,7 @@ NtGdiOffsetViewportOrgEx(
     }
     pdcattr->ptlViewportOrg.x += XOffset;
     pdcattr->ptlViewportOrg.y += YOffset;
-    pdcattr->flXform |= PAGE_XLATE_CHANGED;
+    pdcattr->flXform |= PAGE_XLATE_CHANGED | DEVICE_TO_WORLD_INVALID;
 
     DC_UnlockDc(dc);
 
@@ -921,7 +928,7 @@ GreSetViewportOrgEx(
 
     pdcattr->ptlViewportOrg.x = X;
     pdcattr->ptlViewportOrg.y = Y;
-    pdcattr->flXform |= PAGE_XLATE_CHANGED;
+    pdcattr->flXform |= PAGE_XLATE_CHANGED | DEVICE_TO_WORLD_INVALID;
 
     DC_UnlockDc(dc);
     return TRUE;
@@ -972,7 +979,7 @@ NtGdiSetViewportOrgEx(
 
     pdcattr->ptlViewportOrg.x = X;
     pdcattr->ptlViewportOrg.y = Y;
-    pdcattr->flXform |= PAGE_XLATE_CHANGED;
+    pdcattr->flXform |= PAGE_XLATE_CHANGED | DEVICE_TO_WORLD_INVALID;
 
     DC_UnlockDc(dc);
 
@@ -1024,7 +1031,7 @@ NtGdiSetWindowOrgEx(
 
     pdcattr->ptlWindowOrg.x = X;
     pdcattr->ptlWindowOrg.y = Y;
-    pdcattr->flXform |= PAGE_XLATE_CHANGED;
+    pdcattr->flXform |= PAGE_XLATE_CHANGED | DEVICE_TO_WORLD_INVALID;
 
     DC_UnlockDc(dc);
 
@@ -1060,7 +1067,7 @@ IntMirrorWindowOrg(PDC dc)
     X = (X * pdcattr->szlWindowExt.cx) / cx;
 
     pdcattr->ptlWindowOrg.x = pdcattr->lWindowOrgx - X; // Now set the inverted win origion.
-    pdcattr->flXform |= PAGE_XLATE_CHANGED;
+    pdcattr->flXform |= PAGE_XLATE_CHANGED | DEVICE_TO_WORLD_INVALID;
 
     return;
 }
diff --git a/win32ss/gdi/ntgdi/coord.h b/win32ss/gdi/ntgdi/coord.h
index 9993ef527a..2cb5af0298 100644
--- a/win32ss/gdi/ntgdi/coord.h
+++ b/win32ss/gdi/ntgdi/coord.h
@@ -131,6 +131,9 @@ DC_vXformDeviceToWorld(
     PMATRIX pmx;
 
     pmx = DC_pmxDeviceToWorld(pdc);
+    if (!MX_IsValid(pmx))
+        return;
+
     XFORMOBJ_vInit(&xo, pmx);
     XFORMOBJ_bApplyXform(&xo, XF_LTOL, cNumPoints, pptlDest, pptlSource);
 }
diff --git a/win32ss/gdi/ntgdi/xformobj.c b/win32ss/gdi/ntgdi/xformobj.c
index 0d938df687..ea798f1e7b 100644
--- a/win32ss/gdi/ntgdi/xformobj.c
+++ b/win32ss/gdi/ntgdi/xformobj.c
@@ -243,6 +243,25 @@ XFORMOBJ_iCombineXform(
     }
 }
 
+BOOL FASTCALL
+MX_IsValid(IN PMATRIX pmx)
+{
+    FLOATOBJ foDet;
+    MulSub(&foDet, &pmx->efM11, &pmx->efM22, &pmx->efM12, &pmx->efM21);
+    return !FLOATOBJ_Equal0(&foDet);
+}
+
+VOID FASTCALL
+MX_Set0(OUT PMATRIX pmx)
+{
+    FLOATOBJ_Set0(&pmx->efM11);
+    FLOATOBJ_Set0(&pmx->efM12);
+    FLOATOBJ_Set0(&pmx->efM21);
+    FLOATOBJ_Set0(&pmx->efM22);
+    FLOATOBJ_Set0(&pmx->efDx);
+    FLOATOBJ_Set0(&pmx->efDy);
+}
+
 /*
  * A^-1 = adj(A) / det(AT)
  * A^-1 = 1/(a*d - b*c) * (a22,-a12,a21,-a11)
diff --git a/win32ss/gdi/ntgdi/xformobj.h b/win32ss/gdi/ntgdi/xformobj.h
index 4adeb3d818..4cb9dd3bf9 100644
--- a/win32ss/gdi/ntgdi/xformobj.h
+++ b/win32ss/gdi/ntgdi/xformobj.h
@@ -74,3 +74,6 @@ XFORMOBJ_bApplyXform(
     IN ULONG cPoints,
     IN PVOID pvIn,
     OUT PVOID pvOut);
+
+BOOL FASTCALL MX_IsValid(IN PMATRIX pmx);
+VOID FASTCALL MX_Set0(OUT PMATRIX pmx);