diff --git a/parser.jison b/parser.jison
index d7749e8..0a3d943 100644
--- a/parser.jison
+++ b/parser.jison
@@ -39,6 +39,7 @@
 
 (calc)                                 return 'NESTED_CALC';
 (var\([^\)]*\))                        return 'CSS_VAR';
+([a-z]+\([a-z0-9-]+\))                 return 'UNKNOW_FUNCTION'
 ([a-z]+)                               return 'PREFIX';
 
 "("                                    return 'LPAREN';
@@ -72,6 +73,7 @@ expression
     | css_variable { $$ = $1; }
   	| css_value { $$ = $1; }
   	| value { $$ = $1; }
+    | UNKNOW_FUNCTION { $$ = { type: 'Unknow', value: $1 }; }
     ;
 
   value
diff --git a/src/__tests__/index.js b/src/__tests__/index.js
index 1937587..e1c30af 100644
--- a/src/__tests__/index.js
+++ b/src/__tests__/index.js
@@ -324,3 +324,10 @@ test(
   'calc( (1em - calc( 10px + 1em)) / 2)',
   '-5px'
 )
+
+test(
+  'should not throw an exception when unknow function exist in calc',
+  testFixture,
+  'calc(constant(safe-area-inset-left) + 100px - 50px)',
+  'calc(constant(safe-area-inset-left) + 50px)'
+)
diff --git a/src/lib/stringifier.js b/src/lib/stringifier.js
index c7b14f0..ab604ec 100644
--- a/src/lib/stringifier.js
+++ b/src/lib/stringifier.js
@@ -36,6 +36,7 @@ function stringify(node, prec) {
     case "Value":
       return round(node.value, prec)
     case 'CssVariable':
+    case 'Unknow':
       return node.value
     default:
       return round(node.value, prec) + node.unit