diff --git a/src/DataFilter.cpp b/src/DataFilter.cpp
index 958fc1820..9003857c4 100644
--- a/src/DataFilter.cpp
+++ b/src/DataFilter.cpp
@@ -1084,8 +1084,8 @@ void Leaf::validateFilter(DataFilter *df, Leaf *leaf)
         break;
 
     case Leaf::UnaryOperation :
-        { // only 1 at present, unary minus return the value * -1
-            if (!Leaf::isNumber(df, leaf->lvalue.l)) {
+        { // unary minus needs a number, unary ! is happy with anything
+            if (leaf->op == '-' && !Leaf::isNumber(df, leaf->lvalue.l)) {
                 DataFiltererrors << QString(tr("unary negation on a string!"));
                 leaf->inerror = true;
             }
@@ -2087,7 +2087,15 @@ Result Leaf::eval(Context *context, DataFilter *df, Leaf *leaf, float x, RideIte
     {
         // get result
         Result lhs = eval(context, df, leaf->lvalue.l, x, m, p);
-        return Result(lhs.number * -1);
+
+        // unary minus
+        if (leaf->op == '-') return Result(lhs.number * -1);
+
+        // unary not
+        if (leaf->op == '!') return Result(!lhs.number);
+
+        // unknown
+        return(Result(0));
     }
     break;
 
diff --git a/src/DataFilter.y b/src/DataFilter.y
index 6e7de1b6b..fca405049 100644
--- a/src/DataFilter.y
+++ b/src/DataFilter.y
@@ -117,6 +117,12 @@ lexpr   : expr                       { $$ = $1; }
                                           $$->fparms << $3;
                                           $$->fparms << $5;
                                           $$->op = 0; }
+        | '!' lexpr %prec OR         { $$ = new Leaf(@1.first_column, @2.last_column);
+                                      $$->type = Leaf::UnaryOperation;
+                                      $$->lvalue.l = $2;
+                                      $$->op = '!';
+                                      $$->rvalue.l = NULL; 
+                                     }
 
         | lexpr OR lexpr             { $$ = new Leaf(@1.first_column, @3.last_column);
                                       $$->type = Leaf::Logical;