[acs-r] Warning message

[arilamstein@gmail.com]

When I try to get median age of all ZCTAs I get a warning message that I
don't understand:

acs.data = acs.fetch(geography=geo.make(zip.code = "*"),
                     table.number = "B01002",
                     col.names = "pretty",
                     endyear = 2012,
                     span = 5)

Warning message:
NAs introduced by coercion

The resulting data.frame appears OK to me:

> head(acs.data)
ACS DATA:
 2008 -- 2012 ;
  Estimates w/90% confidence intervals;
  for different intervals, see confint()
            Median Age by Sex: Median age -- Total: Median Age by Sex:
Median age -- Male
ZCTA5 00601 36.4 +/- 0.5                            34.7 +/- 0.9

ZCTA5 00602 38 +/- 0.4                              37.2 +/- 0.5

ZCTA5 00603 38.4 +/- 0.6                            36.7 +/- 0.7

ZCTA5 00606 36.8 +/- 2                              36.8 +/- 3.2

ZCTA5 00610 38.6 +/- 0.6                            36.1 +/- 0.8

ZCTA5 00612 38.1 +/- 0.6                            36.9 +/- 1.2

            Median Age by Sex: Median age -- Female
ZCTA5 00601 37.7 +/- 1.5
ZCTA5 00602 38.7 +/- 0.6
ZCTA5 00603 40.1 +/- 0.6
ZCTA5 00606 36.8 +/- 2.9
ZCTA5 00610 40.5 +/- 0.5
ZCTA5 00612 39.2 +/- 1

Can someone tell me what this warning means, and whether I need to be
concerned about the resulting data?

Thanks.

Ari
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