diff --git a/src/OS/Draw.tcl b/src/OS/Draw.tcl
index 967f97f914..01dab2fa67 100644
--- a/src/OS/Draw.tcl
+++ b/src/OS/Draw.tcl
@@ -17,7 +17,7 @@
 proc Draw:toolkits { } {
     set aResult [list TKDraw TKTopTest TKViewerTest TKXSDRAW TKDCAF TKXDEDRAW TKTObjDRAW TKQADraw]
 
-    if { $::env(HAVE_VTK) == "true" } {
+    if { [info exists ::env(HAVE_VTK)] && $::env(HAVE_VTK) == "true" } {
       lappend aResult "TKIVtkDraw"
     }
 
diff --git a/src/OS/Visualization.tcl b/src/OS/Visualization.tcl
index da793208f5..72b4f2dd29 100644
--- a/src/OS/Visualization.tcl
+++ b/src/OS/Visualization.tcl
@@ -22,7 +22,7 @@ proc Visualization:toolkits { } {
                      TKNIS \
                      TKVoxel]
 
-    if { "$::env(HAVE_VTK)" == "true" } {
+    if { [info exists ::env(HAVE_VTK)] && "$::env(HAVE_VTK)" == "true" } {
       lappend aResult "TKIVtk"
     }
 
@@ -53,7 +53,7 @@ proc Visualization:depends { } {
 proc Visualization:acdepends { } {
     set aList [list X11 GL FREETYPE]
 
-    if { "$::env(HAVE_VTK)" == "true" } {
+    if { [info exists ::env(HAVE_VTK)] && "$::env(HAVE_VTK)" == "true" } {
       lappend aList "VTK"
     }