Warning message: In dist(Data) : NAs introduced by coercion

Thanks. Is there a difference between having NA's in your output and NA's introduced? I ask because when I run the following code it didn't list any:

#Check for NAs
str(Data)

Sample result:

'data.frame':	653 obs. of  16 variables:
 $ Gender            : Factor w/ 2 levels "a","b": 2 1 1 2 2 2 2 1 2 2 ...
 $ Age               : num  30.8 58.7 24.5 27.8 20.2 ...
 $ MonthlyExpenses   : int  0 4 0 1 5 4 1 11 0 4 ...
 $ MaritalStatus     : Factor w/ 3 levels "l","u","y": 2 2 2 2 2 2 2 2 3 3 ...
 $ HomeStatus        : Factor w/ 3 levels "g","gg","p": 1 1 1 1 1 1 1 1 3 3 ...

This is really confusing me....