diff --git a/base/bitset.jl b/base/bitset.jl
index 0abd9d4b782d2..22ff177695349 100644
--- a/base/bitset.jl
+++ b/base/bitset.jl
@@ -137,20 +137,10 @@ function union!(s::BitSet, r::AbstractUnitRange{<:Integer})
 
     # grow s.bits as necessary
     if diffb >= len
-        _growend!(s.bits, diffb - len + 1)
-        # we set only some values to CHK0, those which will not be
-        # fully overwritten (i.e. only or'ed with `|`)
-        s.bits[end] = CHK0 # end == diffb + 1
-        if diffa >= len
-            s.bits[diffa + 1] = CHK0
-        end
+        _growend0!(s.bits, diffb - len + 1)
     end
     if diffa < 0
-        _growbeg!(s.bits, -diffa)
-        s.bits[1] = CHK0
-        if diffb < 0
-            s.bits[diffb - diffa + 1] = CHK0
-        end
+        _growbeg0!(s.bits, -diffa)
         s.offset = cidxa # s.offset += diffa
         diffb -= diffa
         diffa = 0
diff --git a/test/bitset.jl b/test/bitset.jl
index 1919da4f3702a..6c7947ebc37af 100644
--- a/test/bitset.jl
+++ b/test/bitset.jl
@@ -351,3 +351,12 @@ end
     # union! with an empty range doesn't modify the BitSet
     @test union!(x, b:a) == y
 end
+
+@testset "union!(::BitSet, ::AbstractUnitRange) when two ranges do not overlap" begin
+    # see #45574
+    a, b = rand(-10000:-5000), rand(5000:10000)
+    c, d = minmax(rand(20000:30000, 2)...)
+    @test length(union!(BitSet(a:b), c:d)) == length(a:b) + length(c:d)
+    c, d = minmax(rand(-30000:-20000, 2)...)
+    @test length(union!(BitSet(a:b), c:d)) == length(a:b) + length(c:d)
+end