diff --git a/include/sta/Property.hh b/include/sta/Property.hh
index be06b272..c8bf3ab8 100644
--- a/include/sta/Property.hh
+++ b/include/sta/Property.hh
@@ -80,9 +80,9 @@ public:
   const Unit *unit() const { return unit_; }
 
   const char *asString(const Network *network) const;
-  const char *stringValue() const { return string_; }
-  float floatValue() const { return float_; }
-  bool boolValue() const { return bool_; }
+  const char *stringValue() const; // valid for type string
+  float floatValue() const;        // valid for type float
+  bool boolValue() const;          // valid for type bool
   const LibertyLibrary *libertyLibrary() const { return liberty_library_; }
   const LibertyCell *libertyCell() const { return liberty_cell_; }
   const LibertyPort *libertyPort() const { return liberty_port_; }
diff --git a/search/Property.cc b/search/Property.cc
index d51b5303..1477856f 100644
--- a/search/Property.cc
+++ b/search/Property.cc
@@ -126,6 +126,35 @@ PropertyUnknown::what() const noexcept
 
 ////////////////////////////////////////////////////////////////
 
+class PropertyTypeWrong : public Exception
+{
+public:
+  PropertyTypeWrong(const char *accessor,
+                    const char *type);
+  virtual ~PropertyTypeWrong() {}
+  virtual const char *what() const noexcept;
+
+private:
+  const char *accessor_;
+  const char *type_;
+};
+
+PropertyTypeWrong::PropertyTypeWrong(const char *accessor,
+                                     const char *type) :
+  Exception(),
+  accessor_(accessor),
+  type_(type)
+{
+}
+
+const char *
+PropertyTypeWrong::what() const noexcept
+{
+  return stringPrint("property accessor %s is only valid for %s properties.",
+		     accessor_, type_);
+}
+////////////////////////////////////////////////////////////////
+
 PropertyValue::PropertyValue() :
   type_(type_none),
   unit_(nullptr)
@@ -620,6 +649,30 @@ PropertyValue::asString(const Network *network) const
   return nullptr;
 }
 
+const char *
+PropertyValue::stringValue() const
+{
+  if (type_ != Type::type_string)
+    throw PropertyTypeWrong("stringValue", "string");
+  return string_;
+}
+
+float
+PropertyValue::floatValue() const
+{
+  if (type_ != Type::type_float)
+    throw PropertyTypeWrong("floatValue", "float");
+  return float_;
+}
+
+bool
+PropertyValue::boolValue() const
+{
+  if (type_ != Type::type_bool)
+    throw PropertyTypeWrong("boolValue", "boolt");
+  return bool_;
+}
+
 ////////////////////////////////////////////////////////////////
 
 PropertyValue