diff --git a/SOURCE/EXPRPARS.INC b/SOURCE/EXPRPARS.INC
index 1aa69f7..9ccdedb 100644
--- a/SOURCE/EXPRPARS.INC
+++ b/SOURCE/EXPRPARS.INC
@@ -135,8 +135,11 @@ convert_number:
       subexpression_closed:
 	ret
       symbol_value:
-	cmp	[source_start],0
-	je	preprocessor_value
+	mov	eax,[source_start]
+	test	eax,eax
+	jz	preprocessor_value
+	cmp	eax,-1
+	je	invalid_value
 	push	edi esi
 	lods	word [esi]
 	cmp	al,1Ah
diff --git a/SOURCE/VERSION.INC b/SOURCE/VERSION.INC
index 7cea885..644cabe 100644
--- a/SOURCE/VERSION.INC
+++ b/SOURCE/VERSION.INC
@@ -33,7 +33,7 @@
 ; cannot simply be copied and put under another distribution licence
 ; (including the GNU Public Licence).
 
-VERSION_STRING equ "1.73.28"
+VERSION_STRING equ "1.73.29"
 
 VERSION_MAJOR = 1
 VERSION_MINOR = 73
diff --git a/WHATSNEW.TXT b/WHATSNEW.TXT
index 44f10a4..17107a7 100644
--- a/WHATSNEW.TXT
+++ b/WHATSNEW.TXT
@@ -2,6 +2,11 @@
 Visit http://flatassembler.net/ for more information.
 
 
+version 1.73.29 (Dec 18, 2021)
+
+[-] Corrected an issue in external use of expression calculator.
+
+
 version 1.73.28 (Sep 16, 2021)
 
 [+] Added special preprocessing of symbols "__file__" and "__line__"