diff --git a/jackson/src/main/java/eu/mihosoft/vmf/jackson/VMFJsonSchemaGenerator.java b/jackson/src/main/java/eu/mihosoft/vmf/jackson/VMFJsonSchemaGenerator.java
index 0dd1678..810791e 100644
--- a/jackson/src/main/java/eu/mihosoft/vmf/jackson/VMFJsonSchemaGenerator.java
+++ b/jackson/src/main/java/eu/mihosoft/vmf/jackson/VMFJsonSchemaGenerator.java
@@ -497,7 +497,10 @@ private void addConstraint(Property property, Map<String, Object> propertySchema
                             constraintValueToWrite = Double.parseDouble(constraintValue);
                         } catch (NumberFormatException e2) {
                             try {
-                                constraintValueToWrite = Boolean.parseBoolean(constraintValue);
+                                if(constraintValue.equalsIgnoreCase("true")
+                                        || constraintValue.equalsIgnoreCase("false")) {
+                                    constraintValueToWrite = Boolean.parseBoolean(constraintValue);
+                                }
                             } catch (NumberFormatException e3) {
                                 // ignore, not possible to convert to boolean, integer or double, we assume string
                             }