diff --git a/vunit/vhdl/data_types/src/api/external_integer_vector_pkg.vhd b/vunit/vhdl/data_types/src/api/external_integer_vector_pkg.vhd
index d52259488..8cae1038a 100644
--- a/vunit/vhdl/data_types/src/api/external_integer_vector_pkg.vhd
+++ b/vunit/vhdl/data_types/src/api/external_integer_vector_pkg.vhd
@@ -37,11 +37,13 @@ package body external_integer_vector_pkg is
     i  : integer
   ) return integer is begin
     assert false report "EXTERNAL read_integer" severity failure;
+    return integer'low;
   end;
 
   impure function get_ptr (
     id : integer
   ) return extintvec_access_t is begin
     assert false report "EXTERNAL get_intvec_ptr" severity failure;
+    return null;
   end;
 end package body;
diff --git a/vunit/vhdl/data_types/src/api/external_string_pkg.vhd b/vunit/vhdl/data_types/src/api/external_string_pkg.vhd
index 95535c611..5c32472ee 100644
--- a/vunit/vhdl/data_types/src/api/external_string_pkg.vhd
+++ b/vunit/vhdl/data_types/src/api/external_string_pkg.vhd
@@ -37,11 +37,13 @@ package body external_string_pkg is
     i  : integer
   ) return character is begin
     assert false report "EXTERNAL read_char" severity failure;
+    return NUL;
   end;
 
   impure function get_ptr (
     id : integer
   ) return extstring_access_t is begin
     assert false report "EXTERNAL get_string_ptr" severity failure;
+    return null;
   end;
 end package body;
diff --git a/vunit/vhdl/data_types/src/string_ptr_pkg-body-2002p.vhd b/vunit/vhdl/data_types/src/string_ptr_pkg-body-2002p.vhd
index 541d5e1de..8d37498c6 100644
--- a/vunit/vhdl/data_types/src/string_ptr_pkg-body-2002p.vhd
+++ b/vunit/vhdl/data_types/src/string_ptr_pkg-body-2002p.vhd
@@ -285,6 +285,7 @@ package body string_ptr_pkg is
         when others =>
           -- @TODO Implement to_string for external models
           check_external(ref, "to_string");
+          return "";
       end case;
     end;