diff --git a/src/anchoredinterval.jl b/src/anchoredinterval.jl
index 9a66d4a0..da98e9d8 100644
--- a/src/anchoredinterval.jl
+++ b/src/anchoredinterval.jl
@@ -132,7 +132,14 @@ function Base.last(interval::AnchoredInterval{P}) where P
 end
 
 anchor(interval::AnchoredInterval) = interval.anchor
-span(interval::AnchoredInterval{P}) where P = abs(P)
+
+function span(interval::AnchoredInterval{P}) where P
+    if !isunbounded(interval)
+        abs(P)
+    else
+        throw(ArgumentError("Unable to determine the span of an unbounded interval"))
+    end
+end
 
 ##### CONVERSION #####
 
diff --git a/src/interval.jl b/src/interval.jl
index 99aa1ef4..6c4af654 100644
--- a/src/interval.jl
+++ b/src/interval.jl
@@ -123,7 +123,15 @@ end
 
 Base.first(interval::Interval) = interval.first
 Base.last(interval::Interval) = interval.last
-span(interval::Interval) = interval.last - interval.first
+
+function span(interval::Interval)
+    if !isunbounded(interval)
+        interval.last - interval.first
+    else
+        throw(ArgumentError("Unable to determine the span of an unbounded interval"))
+    end
+end
+
 inclusivity(interval::AbstractInterval{T,L,R}) where {T,L,R} = Inclusivity(L === Closed, R === Closed)
 isclosed(interval::AbstractInterval) = isclosed(inclusivity(interval))
 Base.isopen(interval::AbstractInterval) = isopen(inclusivity(interval))