diff --git a/common/libeval_compiler/libeval_compiler.cpp b/common/libeval_compiler/libeval_compiler.cpp
index bfcd3392d3..69081171f8 100644
--- a/common/libeval_compiler/libeval_compiler.cpp
+++ b/common/libeval_compiler/libeval_compiler.cpp
@@ -549,7 +549,8 @@ bool COMPILER::lexDefault( T_TOKEN& aToken )
 
 
         default:
-            reportError( CST_PARSE, wxString::Format( _( "Unrecognized character '%c'" ), (char) ch ) );
+            reportError( CST_PARSE, wxString::Format( _( "Unrecognized character '%c'" ),
+                                                      (char) ch ) );
             break;
         }
 
@@ -979,6 +980,11 @@ bool COMPILER::generateUCode( UCODE* aCode, CONTEXT* aPreflightContext )
             }
             else
             {
+                msg.Printf( _( "Missing units for '%s'| (%s)" ),
+                            *node->value.str,
+                            m_unitResolver->GetSupportedUnitsMessage() );
+                reportError( CST_CODEGEN, msg, node->srcPos );
+
                 value = DoubleValueFromString( EDA_UNITS::UNSCALED, *node->value.str );
             }
 
diff --git a/include/libeval_compiler/libeval_compiler.h b/include/libeval_compiler/libeval_compiler.h
index 134028671d..5175793aa5 100644
--- a/include/libeval_compiler/libeval_compiler.h
+++ b/include/libeval_compiler/libeval_compiler.h
@@ -168,6 +168,11 @@ public:
         return nullUnits;
     }
 
+    virtual wxString GetSupportedUnitsMessage() const
+    {
+        return wxEmptyString;
+    }
+
     virtual double Convert( const wxString& aString, int unitType ) const
     {
         return 0.0;
diff --git a/pcbnew/pcb_expr_evaluator.cpp b/pcbnew/pcb_expr_evaluator.cpp
index 9cafe92452..634a3a03bc 100644
--- a/pcbnew/pcb_expr_evaluator.cpp
+++ b/pcbnew/pcb_expr_evaluator.cpp
@@ -542,6 +542,11 @@ public:
         return pcbUnits;
     }
 
+    virtual wxString GetSupportedUnitsMessage() const override
+    {
+        return _( "must be mm, in, or mil" );
+    }
+
     virtual double Convert( const wxString& aString, int unitId ) const override
     {
         double v = wxAtof( aString );