diff --git a/lib/hamlit/concerns/indentable.rb b/lib/hamlit/concerns/indentable.rb
index 6a90e4d5..b78b8cd1 100644
--- a/lib/hamlit/concerns/indentable.rb
+++ b/lib/hamlit/concerns/indentable.rb
@@ -1,12 +1,12 @@
 require 'hamlit/concerns/error'
 
 module Hamlit
+  EOF = -1
+
   module Concerns
     module Indentable
       include Concerns::Error
 
-      EOF = -1
-
       def reset_indent
         @current_indent = 0
       end
diff --git a/lib/hamlit/parser.rb b/lib/hamlit/parser.rb
index 31c22f5d..1d606e0e 100644
--- a/lib/hamlit/parser.rb
+++ b/lib/hamlit/parser.rb
@@ -47,7 +47,19 @@ module Hamlit
     # Parse lines in current_indent and return ASTs.
     def parse_lines
       ast = []
-      while next_indent == @current_indent
+      loop do
+        indent = next_indent
+        if indent != @current_indent
+          if indent != Hamlit::EOF && indent > @current_indent
+            ast << [:newline]
+            ast << syntax_error(
+              "inconsistent indentation: #{2 * @current_indent} spaces used for indentation, "\
+              "but the rest of the document was indented using #{next_line[/\A +/].to_s.length} spaces"
+            )
+          end
+          break
+        end
+
         @current_lineno += 1
         node = parse_line(current_line)
         if @outer_removal.include?(@current_indent) && ast.last == [:static, "\n"]
diff --git a/spec/hamlit/engine/error_spec.rb b/spec/hamlit/engine/error_spec.rb
index d0beb535..cd6dfd3f 100644
--- a/spec/hamlit/engine/error_spec.rb
+++ b/spec/hamlit/engine/error_spec.rb
@@ -6,5 +6,13 @@ describe Hamlit::Engine do
         "There's no Ruby code for = to evaluate.",
       )
     end
+
+    it 'raises syntax error for illegal indentation' do
+      expect { render_string(<<-HAML.unindent) }.
+        %a
+            %b
+      HAML
+        to raise_error(Hamlit::SyntaxError, 'inconsistent indentation: 2 spaces used for indentation, but the rest of the document was indented using 4 spaces')
+    end
   end
 end