diff --git a/src/__tests__/index.js b/src/__tests__/index.js
index ff6234d..cae4e30 100644
--- a/src/__tests__/index.js
+++ b/src/__tests__/index.js
@@ -1029,3 +1029,24 @@ test(
   'calc(1q + 10pc)',
   '170.33333q'
 );
+
+test(
+  'unknown units',
+  testValue,
+  'calc(1unknown + 2unknown)',
+  'calc(1unknown + 2unknown)'
+);
+
+test(
+  'unknown units with known',
+  testValue,
+  'calc(1unknown + 2px)',
+  'calc(1unknown + 2px)'
+);
+
+test(
+  'unknown units with known (#1)',
+  testValue,
+  'calc(1px + 2unknown)',
+  'calc(1px + 2unknown)'
+);
diff --git a/src/lib/transform.js b/src/lib/transform.js
index 39c78f4..73fc545 100644
--- a/src/lib/transform.js
+++ b/src/lib/transform.js
@@ -19,7 +19,7 @@ function transformValue(value, options, result, item) {
     // stringify calc expression and produce an AST
     const contents = valueParser.stringify(node.nodes);
     const ast = parser.parse(contents);
-    
+
     // reduce AST to its simplest form, that is, either to a single value
     // or a simplified calc expression
     const reducedAst = reducer(ast, options.precision);
diff --git a/src/parser.jison b/src/parser.jison
index d748d69..892cbc1 100644
--- a/src/parser.jison
+++ b/src/parser.jison
@@ -46,6 +46,8 @@
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)\%                return 'PERCENTAGE';
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)\b                return 'NUMBER';
 
+(([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)-?([a-zA-Z_]|[\240-\377]|(\\[0-9a-fA-F]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-fA-F]))([a-zA-Z0-9_-]|[\240-\377]|(\\[0-9a-fA-F]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-fA-F]))*\b       return 'UNKNOWN_DIMENSION';
+
 "("                                             return 'LPAREN';
 ")"                                             return 'RPAREN';
 
@@ -88,6 +90,7 @@ expression
     | TIME { $$ = { type: 'TimeValue', value: parseFloat($1), unit: /[a-z]+$/i.exec($1)[0] }; }
     | FREQ { $$ = { type: 'FrequencyValue', value: parseFloat($1), unit: /[a-z]+$/i.exec($1)[0] }; }
     | RES { $$ = { type: 'ResolutionValue', value: parseFloat($1), unit: /[a-z]+$/i.exec($1)[0] }; }
+    | UNKNOWN_DIMENSION { $$ = { type: 'UnknownDimension', value: parseFloat($1), unit: /[a-z]+$/i.exec($1)[0] }; }
     | EMS { $$ = { type: 'EmValue', value: parseFloat($1), unit: 'em' }; }
     | EXS { $$ = { type: 'ExValue', value: parseFloat($1), unit: 'ex' }; }
     | CHS { $$ = { type: 'ChValue', value: parseFloat($1), unit: 'ch' }; }