handle 2x2 null matrix case

Author: marius311

src/sqrtm.jl

@@ -7,9 +7,13 @@ end
 
 @inline function _sqrtm(::Size{(2,2)}, A::SA) where {SA<:StaticArray}
     a,b,c,d = A
-    s = sqrtm(a*d-b*c)
-    t = inv(sqrtm(a+d+2s))
-    similar_type(SA,typeof(t))(t*(a+s), t*b, t*c, t*(d+s))
+    if a==b==c==d==0
+        zero(A)
+    else
+        s = sqrtm(a*d-b*c)
+        t = inv(sqrtm(a+d+2s))
+        similar_type(SA,typeof(t))(t*(a+s), t*b, t*c, t*(d+s))
+    end
 end
 
 @inline _sqrtm(s::Size, A::StaticArray) = s(Base.sqrtm(Array(A)))