diff --git a/src/auxiliary/math.jl b/src/auxiliary/math.jl
index 3b87c152534..5e5c326edc2 100644
--- a/src/auxiliary/math.jl
+++ b/src/auxiliary/math.jl
@@ -8,7 +8,8 @@
 # AbstractFloat clashes with ForwardDiff.Dual
 sqrt_(x::T) where {T <: Real} = x < zero(x) ? oftype(x, NaN) : Base.sqrt(x)
 # < 0 suffices since log(0) = -Inf
-log_(x::T) where {T <: AbstractFloat} = x < zero(x) ? oftype(x, NaN) : Base.log(x)
+# `AbstractFloat` clashes with `ForwardDiff.Dual` => use `Real`
+log_(x::T) where {T <: Real} = x < zero(x) ? oftype(x, NaN) : Base.log(x)
 
 """
     ln_mean(x, y)