diff --git a/ExportDXF/DrawingInfo.cs b/ExportDXF/DrawingInfo.cs
index 4b6e4b2..f3aed3a 100644
--- a/ExportDXF/DrawingInfo.cs
+++ b/ExportDXF/DrawingInfo.cs
@@ -4,7 +4,7 @@ namespace ExportDXF
 {
     public class DrawingInfo
     {
-        private static Regex drawingFormatRegex = new Regex(@"(?<jobNo>[34]\d{3}(-\d+\w{1,2})?)\s?(?<dwgNo>[ABEP]\d+(-?(\d+|[A-Z]))?)", RegexOptions.IgnoreCase);
+        private static Regex drawingFormatRegex = new Regex(@"(?<jobNo>[34]\d{3}(-\d+\w{1,2})?)\s?(?<dwgNo>[ABEP]\d+(-?(\d+[A-Z]?))?)", RegexOptions.IgnoreCase);
 
         public string JobNo { get; set; }
 
@@ -14,7 +14,7 @@ namespace ExportDXF
 
         public override string ToString()
         {
-            return string.Format("{0} {1}", JobNo, DrawingNo);
+            return $"{JobNo} {DrawingNo}";
         }
 
         public override bool Equals(object obj)
diff --git a/ExportDXF/Forms/MainForm.cs b/ExportDXF/Forms/MainForm.cs
index 223d830..6bd6893 100644
--- a/ExportDXF/Forms/MainForm.cs
+++ b/ExportDXF/Forms/MainForm.cs
@@ -597,16 +597,20 @@ namespace ExportDXF.Forms
             var model = sldWorks.ActiveDoc as ModelDoc2;
             var isDrawing = model is DrawingDoc;
 
-            if (!isDrawing)
-                return;
+            if (isDrawing)
+            {
+                var drawingInfo = DrawingInfo.Parse(activeDocTitleBox.Text);
 
-            var drawingInfo = DrawingInfo.Parse(activeDocTitleBox.Text);
+                if (drawingInfo == null)
+                    return;
 
-            if (drawingInfo == null)
-                return;
-
-            prefixTextBox.Text = string.Format("{0} {1} PT", drawingInfo.JobNo, drawingInfo.DrawingNo);
-            prefixTextBox.SelectionStart = prefixTextBox.Text.Length;
+                prefixTextBox.Text = $"{drawingInfo.JobNo} {drawingInfo.DrawingNo} PT";
+                prefixTextBox.SelectionStart = prefixTextBox.Text.Length;
+            }
+            else
+            {
+                prefixTextBox.Text = string.Empty;
+            }
         }
     }
 }
\ No newline at end of file