https://github.com/Exiv2/exiv2/pull/1486#issuecomment-796843835

src/value.cpp

@@ -411,7 +411,7 @@ namespace Exiv2 {
     {
         value_ = buf;
         // ensure count>0 and nul terminated # https://github.com/Exiv2/exiv2/issues/1484
-        if (value_.size() == 0 || value_[value_.size()-1] != '\0') value_ += '\0';
+        if (value_.size() == 0 || (value_.size() > 0 && value_[value_.size()-1] != '\0')) value_ += '\0';
         return 0;
     }
 

tests/bugfixes/github/test_issue_1484.py

@@ -9,8 +9,20 @@ class test_issue_1484(metaclass=CaseMeta):
     commands  = [ "$exiv2 -g Copyright                            $filename" 
                 , "$exiv2 -M\"set Exif.Image.Copyright Ascii ''\" $filename"
                 , "$exiv2 -g Copyright                            $filename"
+                , "$exiv2 -M\"del Exif.Image.Copyright Ascii\"    $filename"
+                , "$exiv2 -g Copyright                            $filename"
+                , "$exiv2 -M\"set Exif.Image.Copyright\"          $filename"
+                , "$exiv2 -g Copyright                            $filename"
+                , "$exiv2 -M\"set Exif.Image.Copyright me 2021-\" $filename"
+                , "$exiv2 -g Copyright                            $filename"
                 ]
-    stdout   =  ["","","""Exif.Image.Copyright                         Ascii       1  
+    stdout   =  ["","",
+"""Exif.Image.Copyright                         Ascii       1  
+""","","","",
+"""Exif.Image.Copyright                         Ascii       1  
+""","",
+"""Exif.Image.Copyright                         Ascii       9  me 2021-
 """]
     stderr = [""]*len(commands)
-    retval = [ 1,0,0]
+    retval = [1,0,0,0,1,0,0,0,0]    
+