diff --git a/lib/rufo/erb_formatter.rb b/lib/rufo/erb_formatter.rb
index 49cd390d..3f146c45 100644
--- a/lib/rufo/erb_formatter.rb
+++ b/lib/rufo/erb_formatter.rb
@@ -8,7 +8,7 @@ def initialize(src)
   end
 
   def stags
-    ["<%==", "<%+={0,2}"] + super
+    ["<%-", "<%==", "<%+={0,2}"] + super
   end
 
   def etags
diff --git a/spec/lib/rufo/erb_formatter_spec.rb b/spec/lib/rufo/erb_formatter_spec.rb
index 9ad8816c..f00ed0ba 100644
--- a/spec/lib/rufo/erb_formatter_spec.rb
+++ b/spec/lib/rufo/erb_formatter_spec.rb
@@ -99,5 +99,25 @@
       result = subject.format("<% # TODO: fix this later %>")
       expect(result).to eql("<% # TODO: fix this later %>")
     end
+
+    it "handles leading whitespace trim tag" do
+      result = subject.format('<%- if controller_name != "sessions" %>')
+      expect(result).to eql('<%- if controller_name != "sessions" %>')
+    end
+
+    it "handles trailing whitespace trim tag" do
+      result = subject.format('<% if controller_name != "sessions" -%>')
+      expect(result).to eql('<% if controller_name != "sessions" -%>')
+    end
+
+    it "handles both whitespace trim tag" do
+      result = subject.format('<%- if controller_name != "sessions" -%>')
+      expect(result).to eql('<%- if controller_name != "sessions" -%>')
+    end
+
+    it "handles minus unary operator" do
+      result = subject.format("<% - x * y %>")
+      expect(result).to eql("<% -x * y %>")
+    end
   end
 end