diff --git a/src/core/annotation.js b/src/core/annotation.js
index 6246671ad6f30..cd2dd298e6cab 100644
--- a/src/core/annotation.js
+++ b/src/core/annotation.js
@@ -613,8 +613,11 @@ class WidgetAnnotation extends Annotation {
 
     data.readOnly = this.hasFieldFlag(AnnotationFieldFlag.READONLY);
 
-    // Hide signatures because we cannot validate them.
+    // Hide signatures because we cannot validate them, and unset the fieldValue
+    // since it's (most likely) a `Dict` which is non-serializable and will thus
+    // cause errors when sending annotations to the main-thread (issue 10347).
     if (data.fieldType === 'Sig') {
+      data.fieldValue = null;
       this.setFlags(AnnotationFlag.HIDDEN);
     }
   }