diff --git a/HISTORY b/HISTORY
index 6cf9030..e0e2f05 100755
--- a/HISTORY
+++ b/HISTORY
@@ -1,3 +1,6 @@
+Jul. 20 2015
+Fixed a string manipulation bug
+
 Jul. 12 2015
 Added array manipulation functions
 
diff --git a/MY-BASIC Quick Reference.pdf b/MY-BASIC Quick Reference.pdf
index 214a089..25090e4 100755
Binary files a/MY-BASIC Quick Reference.pdf and b/MY-BASIC Quick Reference.pdf differ
diff --git a/core/my_basic.c b/core/my_basic.c
index aa0ad67..65d75fb 100755
--- a/core/my_basic.c
+++ b/core/my_basic.c
@@ -78,7 +78,7 @@ extern "C" {
 /** Macros */
 #define _VER_MAJOR 1
 #define _VER_MINOR 1
-#define _VER_REVISION 57
+#define _VER_REVISION 58
 #define _MB_VERSION ((_VER_MAJOR * 0x01000000) + (_VER_MINOR * 0x00010000) + (_VER_REVISION))
 
 /* Uncomment this line to treat warnings as error */
@@ -1747,10 +1747,11 @@ int _calc_expression(mb_interpreter_t* s, _ls_node_t** l, _object_t** val) {
 	} else {
 		(*val)->type = c->type;
 		if(_is_string(c)) {
-			size_t _sl = strlen(_extract_string(c));
+			char* _str = _extract_string(c);
+			size_t _sl = strlen(_str);
 			(*val)->data.string = (char*)mb_malloc(_sl + 1);
 			(*val)->data.string[_sl] = '\0';
-			memcpy((*val)->data.string, c->data.string, _sl + 1);
+			memcpy((*val)->data.string, _str, _sl + 1);
 		} else if(c->type == _DT_ARRAY) {
 			(*val)->data = c->data;
 			c->type = _DT_NIL;
@@ -2722,6 +2723,9 @@ char* _extract_string(_object_t* obj) {
 	else if(obj->type == _DT_VAR && obj->data.variable->data->type == _DT_STRING)
 		result = obj->data.variable->data->data.string;
 
+	if(!result)
+		result = MB_NULL_STRING;
+
 	return result;
 }
 
diff --git a/output/my_basic.exe b/output/my_basic.exe
index 3c46727..1f712b6 100755
Binary files a/output/my_basic.exe and b/output/my_basic.exe differ
diff --git a/output/my_basic_mac b/output/my_basic_mac
index a32db8f..0027ac9 100755
Binary files a/output/my_basic_mac and b/output/my_basic_mac differ
diff --git a/resource/my_basic.rc b/resource/my_basic.rc
index d28918b..d4739b5 100755
--- a/resource/my_basic.rc
+++ b/resource/my_basic.rc
@@ -36,8 +36,8 @@
 	IDI_ICON_MAIN ICON "icon.ico"
 
 	VS_VERSION_INFO VERSIONINFO
-	FILEVERSION 1,1,57,0
-	PRODUCTVERSION 1,1,57,0
+	FILEVERSION 1,1,58,0
+	PRODUCTVERSION 1,1,58,0
 	FILEFLAGSMASK 0x17L
 #	ifdef _DEBUG
 	FILEFLAGS 0x1L
@@ -55,13 +55,13 @@
 				VALUE "Comments", "MY-BASIC"
 				VALUE "CompanyName", "W. Renxin"
 				VALUE "FileDescription", "MY-BASIC interpreter"
-				VALUE "FileVersion", "1, 1, 57, 0"
+				VALUE "FileVersion", "1, 1, 58, 0"
 				VALUE "InternalName", "my_basic"
 				VALUE "LegalCopyright", "Copyright (C) 2011 - 2015 W. Renxin"
 				VALUE "LegalTrademarks", "MY-BASIC"
 				VALUE "OriginalFilename", "my_basic.exe"
 				VALUE "ProductName", "MY-BASIC"
-				VALUE "ProductVersion", "1, 1, 57, 0"
+				VALUE "ProductVersion", "1, 1, 58, 0"
 			END
 		END
 		BLOCK "VarFileInfo"