diff --git a/prism/src/parser/EvaluateContextConstants.java b/prism/src/parser/EvaluateContextConstants.java
new file mode 100644
index 00000000..c34dc352
--- /dev/null
+++ b/prism/src/parser/EvaluateContextConstants.java
@@ -0,0 +1,59 @@
+//==============================================================================
+//	
+//	Copyright (c) 2018-
+//	Authors:
+//	* Dave Parker <d.a.parker@cs.bham.ac.uk> (University of Birmingham)
+//	
+//------------------------------------------------------------------------------
+//	
+//	This file is part of PRISM.
+//	
+//	PRISM is free software; you can redistribute it and/or modify
+//	it under the terms of the GNU General Public License as published by
+//	the Free Software Foundation; either version 2 of the License, or
+//	(at your option) any later version.
+//	
+//	PRISM is distributed in the hope that it will be useful,
+//	but WITHOUT ANY WARRANTY; without even the implied warranty of
+//	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//	GNU General Public License for more details.
+//	
+//	You should have received a copy of the GNU General Public License
+//	along with PRISM; if not, write to the Free Software Foundation,
+//	Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+//	
+//==============================================================================
+
+package parser;
+
+/**
+ * Information required to evaluate an expression, where no variable value info is available.
+ * Constant values (if needed/present) are stored in a Values object.
+ */
+public class EvaluateContextConstants implements EvaluateContext
+{
+	private Values constantValues;
+
+	public EvaluateContextConstants(Values constantValues)
+	{
+		this.constantValues = constantValues;
+	}
+
+	@Override
+	public Object getConstantValue(String name)
+	{
+		if (constantValues == null)
+			return null;
+		int i = constantValues.getIndexOf(name);
+		if (i == -1)
+			return null;
+		return constantValues.getValue(i);
+	}
+
+	@Override
+	public Object getVarValue(String name, int index)
+	{
+		// No variable info available
+		return null;
+	}
+}
diff --git a/prism/src/parser/ast/ASTElement.java b/prism/src/parser/ast/ASTElement.java
index 17eeb3ff..525869d7 100644
--- a/prism/src/parser/ast/ASTElement.java
+++ b/prism/src/parser/ast/ASTElement.java
@@ -304,7 +304,7 @@ public abstract class ASTElement
 	 */
 	public ASTElement replaceConstants(Values constantValues) throws PrismLangException
 	{
-		return evaluatePartially(new EvaluateContextValues(constantValues, null));
+		return evaluatePartially(new EvaluateContextConstants(constantValues));
 	}
 
 	/**
diff --git a/prism/src/parser/ast/Expression.java b/prism/src/parser/ast/Expression.java
index af13b03c..576d6dff 100644
--- a/prism/src/parser/ast/Expression.java
+++ b/prism/src/parser/ast/Expression.java
@@ -176,7 +176,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public Object evaluate() throws PrismLangException
 	{
-		return evaluate(new EvaluateContextValues(null, null));
+		return evaluate(new EvaluateContextConstants(null));
 	}
 
 	/**
@@ -186,7 +186,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public Object evaluate(Values constantValues) throws PrismLangException
 	{
-		return evaluate(new EvaluateContextValues(constantValues, null));
+		return evaluate(new EvaluateContextConstants(constantValues));
 	}
 
 	/**
@@ -268,7 +268,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public int evaluateInt() throws PrismLangException
 	{
-		return evaluateInt(new EvaluateContextValues(null, null));
+		return evaluateInt(new EvaluateContextConstants(null));
 	}
 
 	/**
@@ -279,7 +279,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public int evaluateInt(Values constantValues) throws PrismLangException
 	{
-		return evaluateInt(new EvaluateContextValues(constantValues, null));
+		return evaluateInt(new EvaluateContextConstants(constantValues));
 	}
 
 	/**
@@ -372,7 +372,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public double evaluateDouble() throws PrismLangException
 	{
-		return evaluateDouble(new EvaluateContextValues(null, null));
+		return evaluateDouble(new EvaluateContextConstants(null));
 	}
 
 	/**
@@ -383,7 +383,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public double evaluateDouble(Values constantValues) throws PrismLangException
 	{
-		return evaluateDouble(new EvaluateContextValues(constantValues, null));
+		return evaluateDouble(new EvaluateContextConstants(constantValues));
 	}
 
 	/**
@@ -467,7 +467,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public boolean evaluateBoolean() throws PrismLangException
 	{
-		return evaluateBoolean(new EvaluateContextValues(null, null));
+		return evaluateBoolean(new EvaluateContextConstants(null));
 	}
 
 	/**
@@ -478,7 +478,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public boolean evaluateBoolean(Values constantValues) throws PrismLangException
 	{
-		return evaluateBoolean(new EvaluateContextValues(constantValues, null));
+		return evaluateBoolean(new EvaluateContextConstants(constantValues));
 	}
 
 	/**
@@ -550,7 +550,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public BigRational evaluateExact() throws PrismLangException
 	{
-		return evaluateExact(new EvaluateContextValues(null, null));
+		return evaluateExact(new EvaluateContextConstants(null));
 	}
 
 	/**
@@ -561,7 +561,7 @@ public abstract class Expression extends ASTElement
 	 */
 	public BigRational evaluateExact(Values constantValues) throws PrismLangException
 	{
-		return evaluateExact(new EvaluateContextValues(constantValues, null));
+		return evaluateExact(new EvaluateContextConstants(constantValues));
 	}
 
 	/**