null - NullReferenceException in F# -

After

When I move through the following code on the second line, the report is empty. . (TaggedResportReportUIVO option) = Report to go = Option.get (taggedResearchReport) if not (report.Equals, the third line is a NullReferenceException

  member generates this.setTaggedResearchReportList (Tap)) then // some stuff here?   
 Why is it so, and what can I do to avoid it? ! Thanks  
 Added later:  
 Here's the call that is  this.setTaggedResearchReportList :  
  getMostRecentTaggedResearchReportForSecurityId (item .id) (New fight & lt; _ & gt; (this.setTaggedResearchReportList 0))   
 Here is the  getMostRecentTaggedResearchReportForSecurityId  method:  
  getMostRecentTaggedResearchReportForSecurityId go (securityId: integer) (callbackUI: action    
 
 
 Your  TaggedResearchReportUIVO  type explicitly F #, And does not allow  null  as a fair value. Therefore, the compiler will prevent you from using the literal  null  in the form of that type of value; However, some other code is going behind the compiler and sticking a zero value there. To work around the instant issue, you can call  Unchecked Diedfolf & lt; TaggedResearchReportUIVO & gt; Instead of  null  you can try to compare.  
 However, to avoid this kind of problem, you should make some more significant changes to the first place. For example, if this  null  is understood as the appropriate value type  TaggedResearchReportUIVO , then you  [& lt; AllowNullLiteral & gt;]  can add attributes in that type of alternative, alternatively, if it is not really understood to use  null  as a fair value, So you have to check the code that is causing the problematic value.  
 On the one hand, there are other parts of your code that can be quite clean. For example,  
  funny (X: option & lt; JSONSingleResult & gt; tag Research done) & gt; & Gt;) - & gt; Match X Really - & gt; Some options (.gate) .result). False - & gt; Anyone   
 to  
  option.map (funny (jsr: JSONSingleResult & lt; _ & gt;) - & gt; jsr.result) < / Code>