diff --git a/regexp.py b/regexp.py
--- a/regexp.py
+++ b/regexp.py
@@ -419,15 +419,14 @@ class RegexpDFA:
 
 		combined_alphabet = set(self.alphabet_index.keys()) | set(alphabet_index.keys())
 		combined_index = {c: i for (i, c) in enumerate(sorted(combined_alphabet))}
-		conversion_index = {v: combined_index[k] for (k, v) in self.alphabet_index.items()}
+		conversion_index = {combined_index[k]: v for (k, v) in self.alphabet_index.items()}
 		n2 = len(combined_alphabet)
 
-		rules = []
-		for i in range(0, len(self.rules), n1):
-			row = ([m]*n2)
-			for (j, st) in enumerate(self.rules[i:i+n1]):
-				row[conversion_index[j]] = st
-			rules.extend(row)
+		rules = [
+			self.rules[i*n1 + conversion_index[j]]
+			if j in conversion_index else m
+			for i in range(m) for j in range(n2)
+		]
 		rules.extend([m]*n2)
 
 		return RegexpDFA(rules, self.end_states, combined_index).reduce().normalize()