diff --git a/src/internal_itp.jl b/src/internal_itp.jl
index af6ee0b97..06f685045 100644
--- a/src/internal_itp.jl
+++ b/src/internal_itp.jl
@@ -30,7 +30,7 @@ function SciMLBase.solve(prob::IntervalNonlinearProblem{IP, Tuple{T, T}}, alg::I
         args...;
         maxiters = 1000, kwargs...) where {IP, T}
     f = Base.Fix2(prob.f, prob.p)
-    left, right = prob.tspan # a and b
+    left, right = minmax(prob.tspan...) # a and b
     fl, fr = f(left), f(right)
     ϵ = eps(T)
     if iszero(fl)
@@ -40,7 +40,7 @@ function SciMLBase.solve(prob::IntervalNonlinearProblem{IP, Tuple{T, T}}, alg::I
         return SciMLBase.build_solution(prob, alg, right, fr;
             retcode = ReturnCode.ExactSolutionRight, left, right)
     end
-    span = abs(right - left)
+    span = right - left
     k1 = T(alg.scaled_k1) / span
     n0 = T(alg.n0)
     n_h = exponent(span / (2 * ϵ))
@@ -49,7 +49,7 @@ function SciMLBase.solve(prob::IntervalNonlinearProblem{IP, Tuple{T, T}}, alg::I
 
     i = 1
     while i ≤ maxiters
-        span = abs(right - left)
+        span = right - left
         mid = (left + right) / 2
         r = ϵ_s - (span / 2)
 
diff --git a/test/internal_rootfinder.jl b/test/internal_rootfinder.jl
index a334eb577..82b0632e7 100644
--- a/test/internal_rootfinder.jl
+++ b/test/internal_rootfinder.jl
@@ -36,4 +36,14 @@ for Rootfinder in (InternalITP,)
         @test abs.(solve(inp3, rf).u) ≈ sqrt.(p)
         @test abs.(solve(inp4, rf).u) ≈ sqrt.(p)
     end
+
+    # https://github.com/SciML/DifferentialEquations.jl/issues/1087
+    # Test with normal and reversed tspan.
+    # The expected zero is -acos(p), but there is another zero at acos(p) just outside tspan
+    f(u, p) = cos(u) - p
+    p = 0.9
+    inp1 = IntervalNonlinearProblem(f, (-1.1 * acos(p), 0.9 * acos(p)), p)
+    inp2 = IntervalNonlinearProblem(f, (0.9 * acos(p), -1.1 * acos(p)), p)
+    @test solve(inp1, rf).u ≈ -acos(p)
+    @test solve(inp2, rf).u ≈ -acos(p)
 end