diff --git a/src/ol/style/expressions.js b/src/ol/style/expressions.js
index b3d2961bb0..40b9774195 100644
--- a/src/ol/style/expressions.js
+++ b/src/ol/style/expressions.js
@@ -115,7 +115,9 @@ export function getValueType(value) {
 /**
  * Context available during the parsing of an expression.
  * @typedef {Object} ParsingContext
- * @property {boolean} inFragmentShader If false, means the expression output should be made for a vertex shader
+ * @property {boolean} [inFragmentShader] If false, means the expression output should be made for a vertex shader
+ * @property {Array<string>} variables List of variables used in the expression; contains **unprefixed names**
+ * @property {Array<string>} attributes List of attributes used in the expression; contains **unprefixed names**
  */
 
 /**
@@ -239,10 +241,14 @@ export const Operators = {
       return ValueTypes.ANY;
     },
     toGlsl: function(context, args) {
-      const prefix = context.inFragmentShader ? 'v_' : 'a_';
       assertArgsCount(args, 1);
       assertString(args[0]);
-      return prefix + expressionToGlsl(context, args[0]);
+      const value = expressionToGlsl(context, args[0]);
+      if (context.attributes.indexOf(value) === -1) {
+        context.attributes.push(value);
+      }
+      const prefix = context.inFragmentShader ? 'v_' : 'a_';
+      return prefix + value;
     }
   },
   'var': {
@@ -252,7 +258,11 @@ export const Operators = {
     toGlsl: function(context, args) {
       assertArgsCount(args, 1);
       assertString(args[0]);
-      return `u_${expressionToGlsl(context, args[0])}`;
+      const value = expressionToGlsl(context, args[0]);
+      if (context.variables.indexOf(value) === -1) {
+        context.variables.push(value);
+      }
+      return `u_${value}`;
     }
   },
   'time': {
diff --git a/test/spec/ol/style/expressions.test.js b/test/spec/ol/style/expressions.test.js
index 1c430d307f..03eb49a896 100644
--- a/test/spec/ol/style/expressions.test.js
+++ b/test/spec/ol/style/expressions.test.js
@@ -114,7 +114,10 @@ describe('ol.style.expressions', function() {
     let context;
 
     beforeEach(function() {
-      context = {};
+      context = {
+        variables: [],
+        attributes: []
+      };
     });
 
     it('correctly converts expressions to GLSL', function() {
@@ -137,6 +140,21 @@ describe('ol.style.expressions', function() {
         'mix(vec4(1.0, 1.0, 1.0, 1.0), vec4(0.0, 0.0, 0.0, 0.0), a_attr4)');
     });
 
+    it('correctly adapts output for fragment shaders', function() {
+      context.inFragmentShader = true;
+      expect(expressionToGlsl(context, ['get', 'myAttr'])).to.eql('v_myAttr');
+    });
+
+    it('correctly adapts output for fragment shaders', function() {
+      expressionToGlsl(context, ['get', 'myAttr']);
+      expressionToGlsl(context, ['var', 'myVar']);
+      expressionToGlsl(context, ['clamp', ['get', 'attr2'], ['get', 'attr2'], ['get', 'myAttr']]);
+      expressionToGlsl(context, ['*', ['get', 'attr2'], ['var', 'myVar']]);
+      expressionToGlsl(context, ['*', ['get', 'attr3'], ['var', 'myVar2']]);
+      expect(context.attributes).to.eql(['myAttr', 'attr2', 'attr3']);
+      expect(context.variables).to.eql(['myVar', 'myVar2']);
+    });
+
   });
 
 });