diff --git a/Library/Homebrew/extend/string.rb b/Library/Homebrew/extend/string.rb
index 3238f97df7..a0cd4d635c 100644
--- a/Library/Homebrew/extend/string.rb
+++ b/Library/Homebrew/extend/string.rb
@@ -17,7 +17,8 @@ module StringInreplaceExtension
   # value with "new_value", or removes the definition entirely.
   def change_make_var! flag, new_value
     new_value = "#{flag}=#{new_value}"
-    gsub! Regexp.new("^#{flag}[ \\t]*=[ \\t]*(.*)$"), new_value
+    sub = gsub! Regexp.new("^#{flag}[ \\t]*=[ \\t]*(.*)$"), new_value
+    opoo "inreplace: changing '#{flag}' to '#{new_value}' failed" if sub.nil?
   end
 
   # Removes variable assignments completely.
@@ -26,7 +27,8 @@ module StringInreplaceExtension
     flags = [flags] unless flags.kind_of? Array
     flags.each do |flag|
       # Also remove trailing \n, if present.
-      gsub! Regexp.new("^#{flag}[ \\t]*=(.*)$\n?"), ""
+      sub = gsub! Regexp.new("^#{flag}[ \\t]*=(.*)$\n?"), ""
+      opoo "inreplace: removing '#{flag}' failed" if sub.nil?
     end
   end
 
diff --git a/Library/Homebrew/utils.rb b/Library/Homebrew/utils.rb
index bb2e7dd838..6c29ba1056 100644
--- a/Library/Homebrew/utils.rb
+++ b/Library/Homebrew/utils.rb
@@ -216,7 +216,11 @@ def inreplace path, before=nil, after=nil
       s.extend(StringInreplaceExtension)
       yield s
     else
-      s.gsub!(before, after)
+      sub = s.gsub!(before, after)
+      if sub.nil?
+        opoo "inreplace in '#{path}' failed"
+        puts "Expected replacement of '#{before}' with '#{after}'"
+      end
     end
 
     f.reopen(path, 'w').write(s)