diff --git a/src/mesa/main/matrix.c b/src/mesa/main/matrix.c
index 0f96f94..97bb18e 100644
--- a/src/mesa/main/matrix.c
+++ b/src/mesa/main/matrix.c
@@ -570,6 +570,8 @@ void
 _mesa_set_viewport( GLcontext *ctx, GLint x, GLint y,
                     GLsizei width, GLsizei height )
 {
+   GLfloat depthMaxF;
+
    if (MESA_VERBOSE & VERBOSE_API)
       _mesa_debug(ctx, "glViewport %d %d %d %d\n", x, y, width, height);
 
@@ -589,6 +591,9 @@ _mesa_set_viewport( GLcontext *ctx, GLint x, GLint y,
    ctx->Viewport.Height = height;
    ctx->NewState |= _NEW_VIEWPORT;
 
+   /* if we don't know the Z buffer's depth at this time, just use 1.0 */
+   depthMaxF = ctx->DrawBuffer ? ctx->DrawBuffer->_DepthMaxF : 1.0F;
+
 #if 1
    /* XXX remove this someday.  Currently the DRI drivers rely on
     * the WindowMap matrix being up to date in the driver's Viewport
@@ -598,7 +603,7 @@ _mesa_set_viewport( GLcontext *ctx, GLint x, GLint y,
                          ctx->Viewport.X, ctx->Viewport.Y,
                          ctx->Viewport.Width, ctx->Viewport.Height,
                          ctx->Viewport.Near, ctx->Viewport.Far,
-                         ctx->DrawBuffer->_DepthMaxF);
+                         depthMaxF);
 #endif
 
    if (ctx->Driver.Viewport) {