diff --git a/eeschema/sch_field.cpp b/eeschema/sch_field.cpp
index 0c4ee8352f..852925c5e2 100644
--- a/eeschema/sch_field.cpp
+++ b/eeschema/sch_field.cpp
@@ -312,6 +312,18 @@ bool SCH_FIELD::IsHorizJustifyFlipped() const
 }
 
 
+bool SCH_FIELD::IsVoid() const
+{
+    // When in UTF-8 mode, wxString puts string iterators in a linked list, and
+    // that linked list is not thread-safe.
+    std::lock_guard<std::mutex> guard( m_mutex );
+
+    size_t len = m_Text.Len();
+
+    return len == 0 || ( len == 1 && m_Text[0] == wxChar( '~' ) );
+}
+
+
 void SCH_FIELD::Place( SCH_EDIT_FRAME* frame, wxDC* DC )
 {
     frame->GetCanvas()->SetMouseCapture( NULL, NULL );
diff --git a/eeschema/sch_field.h b/eeschema/sch_field.h
index dcccc910e6..8adf7526ff 100644
--- a/eeschema/sch_field.h
+++ b/eeschema/sch_field.h
@@ -59,6 +59,8 @@ class SCH_FIELD : public SCH_ITEM, public EDA_TEXT
 
     wxString m_name;
 
+    mutable UNIQUE_MUTEX m_mutex;
+
 public:
     SCH_FIELD( const wxPoint& aPos, int aFieldId, SCH_COMPONENT* aParent,
                const wxString& aName = wxEmptyString );
@@ -113,12 +115,7 @@ public:
      * Function IsVoid
      * returns true if the field is either empty or holds "~".
      */
-    bool IsVoid() const
-    {
-        size_t len = m_Text.Len();
-
-        return len == 0 || ( len == 1 && m_Text[0] == wxChar( '~' ) );
-    }
+    bool IsVoid() const;
 
     void SwapData( SCH_ITEM* aItem ) override;