Merge pull request #681 from TuringLang/mohamed82008-patch-1

Fix #680
TuringLang · Feb 11, 2019 · 1396ae2 · 1396ae2
2 parents 0d5cf23 + 79a8c40
commit 1396ae2
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/src/core/ad.jl b/src/core/ad.jl
@@ -184,7 +184,7 @@ import StatsFuns: nbinomlogpdf
 # Note the definition of NegativeBinomial in Julia is not the same as Wikipedia's.
 # Check the docstring of NegativeBinomial, r is the number of successes and 
 # k is the number of failures
-_nbinomlogpdf_grad_1(r, p, k) = sum(1 / (k + r - i) for i in 1:k) + log(p)
+_nbinomlogpdf_grad_1(r, p, k) = k == 0 ? log(p) : sum(1 / (k + r - i) for i in 1:k) + log(p)
 _nbinomlogpdf_grad_2(r, p, k) = -k / (1 - p) + r / p
 
 nbinomlogpdf(n::Tracker.TrackedReal, p::Tracker.TrackedReal, x::Int) = Tracker.track(nbinomlogpdf, n, p, x)