diff --git a/OpenRA.Game/Support/ConditionExpression.cs b/OpenRA.Game/Support/ConditionExpression.cs
index 8fdb3cd35f..9ffd48ab29 100644
--- a/OpenRA.Game/Support/ConditionExpression.cs
+++ b/OpenRA.Game/Support/ConditionExpression.cs
@@ -111,6 +111,10 @@ namespace OpenRA.Support
 
 		enum TokenType
 		{
+			// fixed values
+			False,
+			True,
+
 			// varying values
 			Number,
 			Variable,
@@ -198,6 +202,12 @@ namespace OpenRA.Support
 					case TokenType.Invalid:
 						yield return new TokenTypeInfo("(<INVALID>)", Precedence.Invalid);
 						continue;
+					case TokenType.False:
+						yield return new TokenTypeInfo("false", Precedence.Value);
+						continue;
+					case TokenType.True:
+						yield return new TokenTypeInfo("true", Precedence.Value);
+						continue;
 					case TokenType.Number:
 						yield return new TokenTypeInfo("(<number>)", Precedence.Value);
 						continue;
@@ -326,6 +336,20 @@ namespace OpenRA.Support
 				return false;
 			}
 
+			static TokenType VariableOrKeyword(string expression, int start, int length)
+			{
+				var i = start;
+				if (length == 4 && expression[i++] == 't' && expression[i++] == 'r' && expression[i++] == 'u'
+						&& expression[i] == 'e')
+					return TokenType.True;
+
+				if (length == 5 && expression[i++] == 'f' && expression[i++] == 'a' && expression[i++] == 'l'
+						&& expression[i++] == 's' && expression[i] == 'e')
+					return TokenType.False;
+
+				return TokenType.Variable;
+			}
+
 			public static TokenType GetNextType(string expression, ref int i, TokenType lastType = TokenType.Invalid)
 			{
 				var start = i;
@@ -438,14 +462,14 @@ namespace OpenRA.Support
 					throw new InvalidDataException("Invalid character '{0}' at index {1}".F(expression[i], start));
 
 				// Scan forwards until we find an invalid name character
-				for (; i < expression.Length; i++)
+				for (i = start; i < expression.Length; i++)
 				{
 					cc = CharClassOf(expression[i]);
 					if (cc == CharClass.Whitespace || cc == CharClass.Operator)
-						return TokenType.Variable;
+						return VariableOrKeyword(expression, start, i - start);
 				}
 
-				return TokenType.Variable;
+				return VariableOrKeyword(expression, start, i - start);
 			}
 
 			public static Token GetNext(string expression, ref int i, TokenType lastType = TokenType.Invalid)
@@ -828,6 +852,18 @@ namespace OpenRA.Support
 							continue;
 						}
 
+						case TokenType.False:
+						{
+							ast.Push(False);
+							continue;
+						}
+
+						case TokenType.True:
+						{
+							ast.Push(True);
+							continue;
+						}
+
 						case TokenType.Number:
 						{
 							ast.Push(Expressions.Expression.Constant(((NumberToken)t).Value));
diff --git a/OpenRA.Test/OpenRA.Game/ConditionExpressionTest.cs b/OpenRA.Test/OpenRA.Game/ConditionExpressionTest.cs
index 1cb61887cb..66cc5faea7 100644
--- a/OpenRA.Test/OpenRA.Game/ConditionExpressionTest.cs
+++ b/OpenRA.Test/OpenRA.Game/ConditionExpressionTest.cs
@@ -23,8 +23,8 @@ namespace OpenRA.Test
 	{
 		IReadOnlyDictionary<string, int> testValues = new ReadOnlyDictionary<string, int>(new Dictionary<string, int>()
 		{
-			{ "true", 1 },
-			{ "false", 0 }
+			{ "one", 1 },
+			{ "five", 5 }
 		});
 
 		void AssertFalse(string expression)
@@ -66,6 +66,26 @@ namespace OpenRA.Test
 			AssertValue("-12", -12);
 		}
 
+		[TestCase(TestName = "Variables")]
+		public void TestVariables()
+		{
+			AssertValue("one", 1);
+			AssertValue("five", 5);
+		}
+
+		[TestCase(TestName = "Boolean Constants")]
+		public void TestBoolConsts()
+		{
+			AssertValue(" true", 1);
+			AssertValue(" true ", 1);
+			AssertValue("true", 1);
+			AssertValue("false", 0);
+			AssertValue("tru", 0);
+			AssertValue("fals", 0);
+			AssertValue("tr", 0);
+			AssertValue("fal", 0);
+		}
+
 		[TestCase(TestName = "Booleans")]
 		public void TestBooleans()
 		{
@@ -320,6 +340,8 @@ namespace OpenRA.Test
 		public void TestUndefinedSymbols()
 		{
 			AssertFalse("undef1 || undef2");
+			AssertValue("undef1", 0);
+			AssertValue("undef1 + undef2", 0);
 		}
 	}
 }