The integral of any probability density from -Inf to Inf should equal 1, correct? I don't understand last result below.

> integrate(function(x) dnorm(x, 0,1), -Inf, Inf)
1 with absolute error < 9.4e-05

> integrate(function(x) dnorm(x, 100,10), -Inf, Inf)
1 with absolute error < 0.00012

> integrate(function(x) dnorm(x, 500,50), -Inf, Inf)
8.410947e-11 with absolute error < 1.6e-10

> all.equal(integrate(function(x) dnorm(x, 500,50), -Inf, Inf)$value, 0)
[1] TRUE

> sessionInfo()
R version 2.10.1 (2009-12-14)
i386-pc-mingw32

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C
[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] statmod_1.4.6      mlmRev_0.99875-1   lme4_0.999375-35   Matrix_0.999375-33 lattice_0.17-26

loaded via a namespace (and not attached):
[1] grid_2.10.1   nlme_3.1-96   stats4_2.10.1 tools_2.10.1

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