Using .iat with Pandas Dataframe

I have two cells - one contains np.nan and the other contains 'example'. I am trying to replace the null cell with a copy of the string, but I keep getting a value error that says

ValueError: could not convert string to float: 'example'.

I don't understand why this needs/wants to convert to a float, how do I just replace the np.nan cell with a copy of the string in the other cell? Thank you!

Edit: it seems that my problem is that I am trying to copy something from a dataframe col with dtype object (the string) and put it into a dataframe col with dtype float64 so that would make my error. I have tried both methods to convert those cols to type object but am not having success. Novice with pandas so I appreciate the patience.

Edit2: The issue was stemming from np.nan not being able to be cast to another type. Fixed, thanks!

        EHR_df.astype({"Scope2_Model": 'object',
                       "Scope2_Serial_Number": 'object',
                       "Scope2_Type": 'object'})
        EHR_df.astype('object', copy=False)
tester1 = EHR_df.iat[hybridInd[i+1], 24] # np.nan cell
tester2 = EHR_df.iat[hybridInd[i+1], 21] # cell containing 'example'
EHR_df.iat[hybridInd[i+1], 24] = tester2 # confused why this does not work
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