diff --git a/win32/Makefile.sub b/win32/Makefile.sub
index 46df464232..aad64ca8f1 100644
--- a/win32/Makefile.sub
+++ b/win32/Makefile.sub
@@ -18,8 +18,11 @@ MFLAGS=-l
 !endif
 
 !if "$(BASERUBY)" == ""
-! if [ruby $(tooldir)/missing-baseruby.bat 2> nul]
-! else if [for %I in (ruby.exe) do @echo BASERUBY = %~s$$PATH:I > baseruby.mk]
+# After `nmake`, just built `ruby.exe` exists in the build directory,
+# and is searched first prior to $PATH.  Assume that no ruby
+# executable in $(tooldir).
+! if [cd $(tooldir) && ruby missing-baseruby.bat 2> nul]
+! else if [(cd $(tooldir) && for %I in (ruby.exe) do @echo BASERUBY = %~s$$PATH:I) > baseruby.mk]
 ! else
 !   include baseruby.mk
 ! endif