diff --git a/CMakeLists.txt b/CMakeLists.txt
index 545c6ca1..6f448347 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -496,6 +496,11 @@ set_property(SOURCE ${STA_SWIG_FILE}
   -I${STA_HOME}/verilog
 )
 
+set_property(SOURCE ${STA_SWIG_FILE}
+  PROPERTY DEPENDS
+  ${STA_HOME}/tcl/Exception.i
+)
+
 swig_add_library(sta_swig
   LANGUAGE tcl
   TYPE STATIC
diff --git a/search/Property.cc b/search/Property.cc
index 549a7a58..4226cc6d 100644
--- a/search/Property.cc
+++ b/search/Property.cc
@@ -92,7 +92,7 @@ PropertyUnknown::PropertyUnknown(const char *type,
 const char *
 PropertyUnknown::what() const noexcept
 {
-  return stringPrint("Error: %s objects do not have a %s property.",
+  return stringPrint("%s objects do not have a %s property.",
 		     type_, property_);
 }
 
diff --git a/search/WritePathSpice.cc b/search/WritePathSpice.cc
index a8c8f2e2..5664ddf6 100644
--- a/search/WritePathSpice.cc
+++ b/search/WritePathSpice.cc
@@ -239,7 +239,7 @@ SubcktEndsMissing::SubcktEndsMissing(const char *cell_name,
 				     const char *subckt_filename) :
   Exception()
 {
-  what_ = "Error: spice subckt for cell ";
+  what_ = "spice subckt for cell ";
   what_ += cell_name;
   what_ += " missing .ends in ";
   what_ += subckt_filename;
diff --git a/tcl/Exception.i b/tcl/Exception.i
index 717e01f0..d4f8cb67 100644
--- a/tcl/Exception.i
+++ b/tcl/Exception.i
@@ -25,7 +25,7 @@
     exit(0);
   }
   catch (std::exception &excp) {
-    Tcl_SetResult(interp, const_cast<char*>(excp.what()), TCL_VOLATILE);
+    Tcl_AppendResult(interp, "Error: ", excp.what(), nullptr);
     return TCL_ERROR;
   }
 }
diff --git a/tcl/Util.tcl b/tcl/Util.tcl
index 0d2bb637..f8f1c929 100644
--- a/tcl/Util.tcl
+++ b/tcl/Util.tcl
@@ -90,7 +90,7 @@ proc check_for_key_args { cmd arg_var } {
   while { $args != "" } {
     set arg [lindex $args 0]
     if { [is_keyword_arg $arg] } {
-      sta_warn "$cmd $arg is not a known keyword or flag."
+      sta_error "$cmd $arg is not a known keyword or flag."
     } else {
       lappend args_rtn $arg
     }
diff --git a/util/Error.cc b/util/Error.cc
index 460af3fb..0215781e 100644
--- a/util/Error.cc
+++ b/util/Error.cc
@@ -58,7 +58,7 @@ FileNotReadable::FileNotReadable(const char *filename) :
 const char *
 FileNotReadable::what() const noexcept
 {
-  return stringPrintTmp("Error: cannot read file %s.", filename_);
+  return stringPrintTmp("cannot read file %s.", filename_);
 }
 
 FileNotWritable::FileNotWritable(const char *filename) :
@@ -69,7 +69,7 @@ FileNotWritable::FileNotWritable(const char *filename) :
 const char *
 FileNotWritable::what() const noexcept
 {
-  return stringPrintTmp("Error: cannot write file %s.", filename_);
+  return stringPrintTmp("cannot write file %s.", filename_);
 }
 
 } // namespace